UC-NRLF 


B    3    mi 


1 


PRACTICAL 
STAMP    MILLING 
AND   AMALGAMATION 


UNIVERSITY  OF  CALIFORNIA. 


Class 


Practical 

Stamp  Milling  and 
Amalgamation 


BY 

H.  W.  MACFARREN 


SECOND    EDITION 

(Second  Printing) 


Published  by  the 
MINING  AND  SCIENTIFIC  PRESS,  SAN  FRANCISCO, 

and 

THE  MINING  MAGAZINE,  LONDON. 
1910. 


COPYRIGHTED  1910 

BY 
DEWEY  PUBLISHING  COMPANY. 


PREFACE 

When  I  first  engaged  in  stamp-milling  and  amalgama- 
ting, like  many  others  I  eagerly  but  unsuccessfully  sought 
for  a  treatise  that  would  tell  me  what  and  how  to  do  and 
why;  that  would  give  the  ideas  and  principles  by  which 
millmen  were  to  be  guided,  and  the  methods  found  to  be 
most  satisfactory.  Later  the  subject  of  examining  ores 
and  the  adjusting  of  the  stamp-mill  to  their  requirements 
became  of  interest.  This  book  is  an  attempt  to  give  in  a 
brief,  concise  way,  information  on  these  points. 

It  incorporates  my  own  experience  and  conclusions, 
and  knowledge  gathered  from  other  millmen  and  metal- 
lurgists. It  leaves  theoretical  discussions  and  academic 
distinctions  aside,  and  goes  straight  to  the  details  of 
practical  work. 

H.  W.  MACFARREN. 
Salt  Lake  City,  July  1,  1910. 


235597 


TABLE  OF  CONTENTS 


CHAPTER  I 

Page. 

The   Stamp-mill    9 

Location  and  Design    11 

Rock-breaker,   Grizzly,   and   Ore-bin 14 

Battery-frame  and  Line-shaft   17 

Mortar-block    18 

CHAPTER  II 

Mortar 22 

Die    23 

Shoe    25,  27 

Bosshead    29,  30 

Tappet    30 

Cam    34 

CHAPTER  III 

Stem    36,  39 

Stem  Crystallization  and  Breakage 36 

Adjusting  Height  of  Drop 44 

Cam  Shaft  and  Cam-shaft  Box 45 

Order  of  Drop 49 

Guide 51 

Finger  Jack  52 

Feeder    52 

Screen    53 

CHAPTER  IV 

Water  Supply   57 

Height  and  Speed  of  Drop 59 

Weight  of  Stamp  60 

Height  of  Discharge  62 

Feeding  the  Mortar 64 

Power    65 

Individual    Stamp    67 


2  TABLE  OF  CONTENTS 

CHAPTER  V 

Page. 

Nature  and  Properties  of  Mercury 71 

Impure  Mercury  and  Its  Care 72 

Principles  and  Practice  of  Amalgamation: 

Amalgamation    74 

Inside  Plates  76 

CHAPTER  VI 
Principles  and  Practice  of  Amalgamation,  Continued: 

Outside   Plates    81 

Outside  Amalgamation  98 

Amalgamation  in  Cyanide  Solution 100 

CHAPTER  VII 
Principles  and  Practice  of  Amalgamation,  Continued: 

Silvered  or  Raw  Copper  Plates  and  Their  Handling 101 

Temperature  of  Battery  Water 106 

Period  Between  Plate  Dressings . .  107 

Unsatisfactory  Hard  Plates 108 

Plates  Away  from  Mortar 108 

Tools,  and  Chemicals  and  Their  Use 110 

Clean-up  and  Cleaning  Amalgam 116 

CHAPTER  VIII 

Retorting  and  Percentage  of  Metal  in  Retort 121 

Melting  and  Sampling  Bullion . .  124 

Treatment  of  Slag,  Old  Screens,  etc 128 

CHAPTER  IX 

Loss  of  Gold  in  Amalgamation  and  Its  Remedies 130 

Sizing  and  Mill  Tests,  and  Sampling 136 

Milling  Systems   142 

CHAPTER  X 

Millmen  and  Mill  Crews 148 

Mill  Management  153 

Handling  Pulp  and  Tailing .158 


Practical 

Stamp    Milling   and 
Amalgamation 


PART  I 

STAMP  MILLING 
CHAPTER  I 

The  supremacy  of  the  stamp-mill  for  crushing  gold  ores 
and  the  reason  why  it  has  so  successfully  withstood  the 
attempts  of  both  theoretical  and  practical  men  to  super- 
sede it  or  limit  its  application,  lie  mainly  in  its  sim- 
plicity, reliability,  and  wide  range  of  adaptability.  The 
modern  gravity-stamp  has  been  evolved  from  the  mortar 
and  pestle  used  by  primitive  man,  and  it  has  remained 
simple  in  construction. 

At  first  sight  it  appears  to  be  a  crude  machine.  All 
is  done  by  gravity.  There  is  an  absence  of  elevating, 
conveying,  and  re-working,  which  give  so  much  trouble 
in  the  other  systems  of  milling.  Its  range  of  adaptability 
is  far  greater  than  that  of  any  other  crusher.  It  is  used 
in  Gilpin  county,  Colorado,  in  the  treatment  of  a  gold 
ore  slow  to  amalgamate,  and  where  its  crushing  capacity 
is  subordinated  in  the  effort  to  give  the  ore  the  particular 
treatment  required  to  save  the  gold,  the  daily  output 
being  reduced  to  one  ton  per  stamp.  In  South  African 
practice  amalgamation  is  subordinated  to  crushing,  re- 
sulting in  a  capacity  as  high  as  ten  tons  per  stamp,  or 
higher.  The  stamp  is  used  for  disintegrating  cemented 
gold-bearing  gravel  and  for  pulverizing  the  softest  rock, 
as  well  as  for  crushing  the  hardest  and  toughest  quartz. 
It  may  be  fed  with  fine  material  having  only  sufficient 
grit  to  keep  the  shoes  from  hammering  the  dies,  up  to 


10  STAMP  MILLING  AND  AMALGAMATION 

slabs  of  rock  the  size  of  a  large  meat-platter  and  3  to 
4  in.  thick.  Such  rock  is  often  sent  through  the  battery 
during  a  break-down  of  the  rock-crusher.  It  will  crush 
wet  or  dry;  it  will  deliver  its  product  through  a  4  or  a 
40-mesh  screen,  or  through  a  still  finer  one  if  desired, 
though  the  modern  stamp-battery  is  not  adapted  to  crush- 
ing to  advantage  through  a  screen  finer  than  40-mesh. 
No  machine  can  compare  with  it  in  amalgamating,  or  in 
preparing  an  ore  for  concentration,  except  where  the  fri- 
able nature  of  the  material  requires  stage-crushing.  In 
the  hands  of  a  metallurgist  who  has  made  a  study  of 
the  stamp-mill,  a  great  diversity  of  treatment  can  be 
given  an  ore  in  an  experimental  way,  to  ascertain  the 
best  for  adoption  in  that  particular  case. 

Estimates  of  the  tonnage  and  costs  with  a  stamp-mill 
can  be  made  in  advance  with  a  close  approximation  to 
exactness.  So  thoroughly  can  it  be  depended  upon  that 
it  has  passed  into  an  axiom  that  a  standard  stamp-mill 
has  never  caused  the  closing  of  a  property  by  failing  to 
do  good  work  where  good  work  was  possible,  and  that 
it  has  replaced  to  advantage,  at  some  time  or  other,  prac- 
tically every  other  kind  of  crushing  machine.  It  is  true 
that  there  are  stamp-mills  that  are  incapable  of  good 
work,  due  to  careless  manufacture,  to  incompetent  mill- 
wrighting,  and  foolish  economizing  on  the  part  of  the 
company;  but  despite  this  the  average  millman  manages 
to  treat  a  fair  tonnage  and  to  make  good  extraction  with 
them,  so  that  the  president  of  the  company  in  his  city 
office  is  often  unaware  that  his  mill  is  not  up  to  the 
standard. 

The  opponents  of  the  stamp-mill  present  elaborate 
statements  showing  that  the  cost  of  installation  and  the 


ADVANTAGES  OF  THE  STAMP  MILL  11 

power  consumed  exceeds  that  of  other  processes.  While 
this  is  often  true  in  certain  cases,  an  average  will  show 
the  balance  to  be  in  favor  of  the  stamp-mill.  It  must 
be  remembered  that  exact  and  correct  data  of  what  a 
stamp-mill  will  do  can  be  compiled,  but  that  the  advance 
estimates  of  what  other  processes  and  devices  will  attain 
is  seldom  realized  under  normal  conditions.  It  is  in  the 
lower  cost  of  operating  and  repairing,  less  loss  of  operat- 
ing time,  and  wider  range  of  treatment  at  command,  that 
the  stamp-mill  overshadows  all  other  crushing  machines 
and  processes. 

It  is  an  object  lesson  to  go  into  some  of  the  large  and 
small  mills  that  stud  the  Mother  Lode  of  California  so 
closely  that  he  who  travels  through  the  counties  of  Ama- 
dor,  Calaveras,  and  Tuolumne,  is  seldom  out  of  hearing 
of  their  roar;  to  note  the  ease  with  which  the  ore  by 
gravity  runs  through  the  mill,  the  little  labor  necessary, 
the  cleanliness  of  the  place,  and  the  smallness  of  the 
scrap  pile,  consisting  mainly  of  worn-out  shoes,  dies,  and 
screens;  and  then  to  go  to  a  wet-roll  or  dry-process 
mill  with  sloppy,  muddy  floors,  or  dust-covered  machinery, 
jammed  rolls,  elevators  out  of  order,  a  small  army  of 
mechanics  and  helpers  on  construction  and  repairs,  and 
mountainous  scrap-pile  of  broken  and  worn-out  machinery. 

The  selection  of  a  mill-site  is  in  some  cases  a  simple 
matter,  in  others  it  is  complicated  by  so  many  factors 
as  to  be  a  most  difficult  problem.  Erecting  the  mill  some 
distance  away  when  it  could  have  been  built  to  advan- 
tage near  the  mine,  is  one  of  the  most  common  mistakes. 
Operations  should  be  centralized  and  concentrated  as 
much  as  possible.  Two  operating  points,  with  much  of 
their  equipment  in  duplicate,  increases  the  working  costs. 


12  STAMP  MILLING  AND  AMALGAMATION 

The  great  cost  of  installing  and  repairing  transportation 
lines  is  often  overlooked  in  making  estimates  of  working 
costs.  Another  common  mistake  is  the  hauling  of  ore  a 
long  distance  to  water,  when  water  could  be  obtained 
close  at  hand  by  a  little  development,  or  by  further  sink- 
ing in  the  mine  if  the  mine  water  be  suited  to  amalgama- 
tion. In  a  few  cases  it  has  not  been. 

Where  ore  is  supplied  to  a  large  mill  by  an  aerial  tram- 
way, an  attempt  should  be  made  to  place  the  mill  so 
that  the  cable  may  run  lengthwise  over  the  bin,  enabling 
the  buckets  to  be  tripped  at  any  point,  thus  dispensing 
with  a  belt-conveyor.  With  a  mill  adjacent  to  a  working 
adit,  straight  tracks  and  heavy,  well-ballasted  rails  will 
enable  large  cars  (3  tons)  to  be  used  with  ease.  At  a 
shaft-mine  it  is  well  to  place  the  mill  near  the  hoist  when 
possible.  Hoisting  may  then  be  done  by  self-dumping 
skips  into  an  ore-bin  from  which  the  ore  may  run  by  grav- 
ity to  one  breaker  for  a  20-stamp  mill,  to  two  breakers, 
one  on  each  side  of  the  bin,  for  a  40-stamp  mill,  or  through 
one  rock-breaking  system  connected  by  belt-conveyor  to 
more  than  40  stamps.  The  objection  to  putting  the  mill 
and  hoist  together  is  that  in  event  of  fire,  both  will  proba- 
bly be  burned,  and  the  fire  communicated  to  the  shaft 
timbers. 

Efforts  should  be  made  to  dispense  with  elevators  for 
both  wet  and  dry  material,  and  to  a  lesser  extent  with 
belt-conveyors.  A  large  part  of  the  advantage  from  using 
the  stamp-battery  is  due  to  the  absence  of  elevating  and 
conveying  machinery.  Extra  expense  to  secure  greater 
simplicity  is  money  well  spent.  The  stamp-battery  is  an 
ideal  illustration  of  the  'unit  idea'  in  construction  as  well 
as  in  operation,  and  a  mill  should  be  so  situated  that  it 


DISPOSAL  OF  TAILING  13 

can  be  added  to,  though  the  proportion  of  mills  that  are 
increased  in  size  is  small. 

The  disposal  of  the  tailing  should  be  carefully  consid- 
ered, and  the  title  or  irrevocable  right  to  ground  on  which 
it  can  be  dumped  should  be  secured,  even  if  it  does  not 
appear  that  this  ground  will  be  needed,  for  a  trespassing 
tailing,  like  smelter-fume,  can  be  made  a  basis  for  dam- 
age suits.  It  is  not  necessary  to  build  the  mill  adjoining 
a  good  site  for  impounding  the  tailing,  or  an  existing 
cyanide  plant.  The  tailing  can  be  ditched,  piped,  or 
flumed  a  long  distance.  Concentrate  has  been  conducted 
in  pipes  as  small  as  1  in.  diam.  down  mountain  sides  and 
across  rivers  to  reduction  works  and  storage  bins. 

Where  it  is  necessary  to  set  a  water-wheel  so  low  that 
it  may  be  in  danger  during  high  water,  in  the  effort  to 
secure  higher  head,  the  mill  may  be  set  higher  up,  out 
of  the  danger  zone,  and  connected  with  the  water-wheel 
by  a  rope-drive. 

The  roof  of  the  mill  should  not  be  in  one  solid  sheet, 
but  should  be  broken  by  drops  at  the  different  floors, 
that  these  floors  may  be  well  lighted  by  windows  set  in 
these  drops. 

It  should  be  possible  to  reach  the  top  and  foot  of  the 
mill  by  wagon,  so  as  to  unload  shoes  and  dies  at  the 
plate-floor,  and  stems  and  other  parts  at  the  cam-floor. 

The  common  practice  in  rock  breaking  is  to  use  a  Blake 
jaw-crusher  for  10  stamps,  a  Blake  or  gyratory  for  20, 
or  two  for  40,  as  one  breaker  cannot  readily  spread  the 
ore  to  more  than  20  stamps.  A  single  rock-breaking  unit 
is  frequently  used  for  40  stamps  and  always  for  more. 
Such  a  unit  usually  consists  of  one  large  coarse-crushing 
gyratory,  which  may  or  may  not  be  preceded  by  a  grizzly. 


14  STAMP  MILLING  AND  AMALGAMATION 

followed  by  one  or  two  fine-crushing  gyratories.  In  the 
latter  case  a  revolving  screen  or  trommel  is  commonly 
placed  between  the  coarse  and  fine-crushers.  The  product 
is  delivered  to  a  belt-conveyor,  if  the  crusher  building  is 
adjacent  to  the  mill,  for  distribution  to  the  bins,  or  it  is 
dropped  into  a  storage-bin  for  conveyance  by  other  means. 

For  breaking  ore  at  a  mill  containing  20  stamps  or  less, 
the  Blake  jaw-crusher  is  the  cheapest  in  cost  and  opera- 
tion; for  more  than  20  stamps,  the  gyratory  is  the  best 
by  reason  of  the  high  capacity  of  the  larger  sizes.  The 
breaker  should  be  driven  by  separate  power  that  it  may 
in  no  way  interfere  with  the  running  of  the  stamps,  and 
should  be  enclosed  from  the  balance  of  the  mill  that  the 
dust  may  be  kept  out  of  the  bearings  and  machinery  below. 

Many  small  mills  are  built  in  which  it  is  necessary  for 
the  breakerman  to  shovel  or  scrape  into  the  breaker 
every  pound  of  ore  that  passes  through  it.  If  sufficient 
height  be  not  available  for  placing  a  crude-ore  bin  above 
the  breaker,  then  a  gyratory  breaker  preceded  by  a  grizzly 
should  be  used,  and  the  ore  should  be  dumped  directly 
upon  the  breaker.  This  method  of  dumping  directly  and 
dispensing  with  a  breakerman  will  not  be  successful  with 
a  wet,  sticky  ore  that  'balls  up'  in  the  breaker,  nor  where 
the  ore-supply  does  not  come  in  small  lots,  although  the 
breaker  may  be  buried  with  a  dry,  brittle  ore  having  no 
lumps  too  large  to  enter  the  jaws. 

Where  it  is  not  desired  to  dump  directly  upon  the 
breaker,  a  crude-ore  bin  should  be  used.  This  need  not 
have  a  sloping  bottom,  for  should  the  supply  run  low, 
the  breakerman  can  enter  the  bin  and  shovel  the  ore 
forward.  The  grizzly  should  follow  this  bin,  emptying 
directly  into  the  crusher.  An  apron  underneath  the 


HEIGHT  OF  DROP  15 

grizzly  should  carry  the  fine  ore  that  has  passed  the  grizzly 
bars  to  the  point  where  the  breaker  discharges,  that  the 
coarse  and  fine  ore  may  be  well  mixed.  The  placing  of 
the  grizzly  before  the  crude-ore  bin  where  the  ore  from 
the  mine  is  dumped  directly  upon  it,  and  the  fine  ore 
shunted  by  the  crude-ore  bin,  or  any  construction  that 
tends  to  keep  the  ore  in  crushed-ore  bin  from  being  homo- 
geneous, is  bad  and  must  be  condemned. 

The  height  of  drop  of  the  stamps  in  a  battery,  and  the 
other  adjustments,  are  made  for  each  ore  according  to  char- 
acter, hardness,  and  fineness.  Where  the  construction  is  such 
that  the  fine  and  coarse  ore  during  the  day  and  early 
evening  become  segregated,  a  hard,  coarse  rock  from  the 
breaker  is  at  first  fed  to  the  mortars,  and  the  mill  works 
splendidly;  but  late  in  the  evening,  the  coarse  rock  in 
the  front  part  of  the  bin  being  exhausted,  the  fine  from 
the  grizzly  begins  to  come.  The  stamps  having  a  long 
drop  for  the  hard,  coarse  rock,  now  sink  through  this  fine 
ore  and  strike  the  dies,  and  from  there  on  the  millman 
has  trouble.  While  the  stamp-battery  can  be  adjusted  to 
work  well  on  this  fine  material,  it  is  obvious  that  such 
adjustments  for  the  two  classes  of  ore  cannot  be  made 
economically  twice  daily.  Besides  the  trouble  in  feeding, 
there  is  danger  in  the  'camming'  of  the  stamps  that  break 
through  the  bed  of  pulp  to  the  die.  This  fine  rock  is 
sometimes  many  times  richer  than  the  coarse,  and  the 
millman,  engrossed  in  other  troubles,  may  fail  to  feed  the 
additional  quicksilver  or  make  the  extra  dressings  of  the 
plates  that  may  be  required. 

The  crushed-ore  bin,  in  fact  any  bin  supplying  an  auto- 
matic feeder,  should  have  a  sloping  bottom  of  from  40 
to  50°.  The  only  real  argument  in  favor  of  the  flat-bottom 


16  STAMP  MILLING  AND  AMALGAMATION 

bin  is  that  it  gives  a  reserve  ore-supply,  but  this  reserve 
might  as  well  be  outside  the  mill,  except  in  the 'case  of 
accident  to  the  breaker,  for  it  will  all  have  to  be  shoveled. 
It  has  been  said  that  this  extra  ore  by  its  weight  anchors 
the  bin  and  adjacent  parts  of  the  mill.  The  reply  to  this 
is  that  a  well-constructed  mill  does  not  require  such 
anchoring.  Many  arguments  and  sophistries  are  advanced 
that  these  flat-bottom  bins  will  be  kept  full  and  that  no 
shoveling  will  be  required;  but  actual  observation  shows 
that  they  are  not  kept  full,  and  that  occasional  shoveling 
must  be  done.  This  requires  an  extra  man  or  extra  men, 
and  the  charges  for  this  item  soon  amount  to  15  to  30  cents 
per  ton.  If  a  millman  has  to  go  into  the  bin,  he  gets 
surly,  and  voluntarily  or  involuntarily  neglects  his  other 
work.  A  sloping  bottom  is  advantageous  where  a  wet, 
fine  ore  that  will  neither  roll  nor  run  is  being  milled,  such 
as  'old  filling*  from  the  mine-stopes.  By  introducing  a 
few  pails  of  water  at  the  top  and  back  of  the  bin,  the 
whole  mass  can  be  started  moving  slowly  into  the  feed- 
chutes;  care  must  be  exercised  in  the  amount  of  water 
used,  or  the  whole  mass  will  move  down  into  the  mortars 
regardless  of  the  feeders,  and,  unless  the  stamps  are  im- 
mediately hung  up,  the  screens  will  be  broken  out;  in 
either  case  the  excess  of  pulp  or  ore  will  have  to  be  dug 
out  of  the  mortars.  In  some  mills  1-in.  pipes  are  arranged 
to  deliver  a  constant  small  stream  of  water  upon  such 
ore  at  the  chute  between  the  bin  and  the  hopper  of  the 
feeder. 

A  compromise  bin  where  the  sloping  bottom  begins  not 
at  the  feed-chute  door,  but  halfway  back  on  the  bin-sills, 
does  not  give  the  advantage  of  either  the  sloping  or  flat- 
bottom  bin.  The  bin  should  have,  in  addition  to  its  double 


POSITION  OF  LINE  SHAFT  17 

planking,  steel  plates  at  the  points  of  greatest  wear,  the 
grizzly  apron,  the  point  where  the  ore  drops  into  the  bin, 
and  just  above  each  feed-chute  opening.  The  lower  3  ft. 
of  the  planking  on  the  front  of  the  bin  should  be  placed 
on  the  outside  of  the  bin-posts,  instead  of  inside,  as  the 
remaining  planking.  This  will  permit  the  millman  to  in- 
sert a  bar  or  shovel  from  the  cam-floor  and  to  start  the 
ore  when  it  is  low  or  has  'bridged',  or  it  will  enable  him 
easily  to  enter  the  bin;  a  cover  of  canvas,  or  a  hinged 
board,  between  the  outside  and  inside  planking  will  keep 
the  dust  back. 

It  is  possible  to  increase  the  size  of  the  mill  by  taking 
a  feed-chute  out  of  the  corner  of  the  bin  and  at  an  angle 
to  it,  to  another  5-stamp  battery  in  line  with  the  original 
batteries.  One  side  of  the  mill  is  always  free  to  make 
such  an  addition,  while  the  other  will  usually  require  some 
change  in  the  driving  arrangements.  Where  this  idea  is 
kept  in  view  in  building  a  10-stamp  mill,  and  a  large  high 
bin  is  built,  it  will  be  possible  to  turn  it  into  a  20-stamp 
easily  and  cheaply. 

In  battery  construction,  the  back-knee  type,  where  the 
battery-posts  are  tied  to  the  ore-bin,  and  the  line-shaft 
driving  the  cam-shafts  is  placed  on  the  streak-sills  under- 
neath the  feeder-floor,  is  now  given  the  preference.  This 
style  requires  the  least  amount  of  timber.  It  is  the  strong- 
est and  most  rigid  construction,  especially  in  view  of  the 
belt  pull.  It  gives  a  clean-cut  and  well-lighted  mill,  with 
the  upper  part  of  the  battery  and  mill  in  sight  from  the 
plate-floor.  The  objections  are  that  a  belt-tightener  must 
be  used  on  the  battery  belts,  but  this  answers  for  the 
friction-clutch  with  which  the  pulleys  of  all  horizontal 
battery  belts  should  be  provided.  However,  the  wear  and 


18  STAMP  MILLING  AND  AMALGAMATION 

tear  on  these  belts  is  much  greater  than  on  belts  not  re- 
quiring tighteners.  It  has  been  feared  that  tying  the 
battery-posts  and  framing  to  the  ore-bin  would  cause  them 
to  be  thrown  out  of  line  by  settling  of  the  bin;  this  can 
only  occur  with  bins  set  on  a  loose,  poor  foundation,  and 
has  seldom  given  any  trouble.  Placing  the  line-shaft  on 
the  streak-sills  has  been  criticized  as  putting  it  in  a  place 
hard  to  get  at  and  subject  to  dirt  and  water.  Plenty  of 
head-room  should  be  allowed  between  the  streak-sills  and 
the  feeder-floor,  that  the  shaft  may  be  easily  reached  for 
repair.  Ring  oilers  should  be  used  and  dust-caps  of  can- 
vas provided.  In  a  well-constructed  mill,  no  water  will 
reach  the  shaft.  The  placing  of  the  line-shaft  on  the  bin- 
sills  in  the  rear  of  a  sloping-bottom  ore-bin  gives  all  the 
advantages  of  the  style  previously  described,  with  the  ad- 
ditional one  that  the  belt  is  horizontal  and  requires  no 
tightener.  This  style  is  limited  to  20  stamps  as  the  bat- 
tery belts  must  pass  on  the  outside  of  the  bin,  and  it  is 
advisable  to  make  the  bins  continuous. 

In  the  front-knee  type,  the  line-shaft  is  placed  on  a  level 
with  the  cam-shaft  and  in  front  of  it  on  heavy  timbers 
that  form  a  part  of  the  battery  framing  and  brace  it  in- 
dependently of  the  ore-bin,  though  it  can  be  tied  to  the 
bin.  It  requires  more  timber,  is  neither  as  strong  nor  as 
steady  as  the  back-knee  type,  while  the  upper  platform 
darkens  the  mill  and  does  not  allow  the  upper  part  of 
the  mill  and  battery  to  be  seen  from  the  plate-floor.  It 
is  not  suitable  for  10  stamps  unless  tied  to  the  bin,  as 
the  tendency  of  the  battery  is  to  sway  endwise  on  account 
of  its  comparatively  small  area  of  longitudinal  anchorage. 

The  subject  of  battery  foundations  is  important,  and  the 
seal  of  approval  has  been  placed  on  concrete  mortar- 


CONCRETE  MORTAR  BLOCKS  19 

blocks;  but  while  realizing  that  a  good  concrete  block 
may  be  perfect,  it  must  be  admitted  that  in  a  large  num- 
ber of  cases  they  have  been  unsatisfactory  from  defective 
construction,  while  the  wooden  block  has  given  satisfac- 
tion in  practically  every  instance.  The  concrete  block 
gives  a  cleaner  looking  mill,  and  does  not  decay.  By  its 
solidity  and  non-cushioning  effect,  it  increases  the  capac- 
ity of  a  battery  sometimes  as  much  as  33%%.  It  is  reputed 
to  break  stems  and  cam-shafts  faster  than  the  wooden 
block,  but  where  the  block  has  been  built  and  the  battery- 
posts  assembled  in  such  manner  that  everything  is  bolted 
tightly  and  securely  together,  and  kept  so,  and  the  jar 
and  vibration  reduced  to  a  minimum,  the  breakage  of 
parts  is  greatly  lessened. 

The  principal  troubles  with  concrete  blocks  have  been : 
a  rocking  of  the  block,  due  to  imperfect  setting  on  the 
bedrock  or  to  too  small  a  base-area  when  set  on  loose 
ground;  a  disintegration  of  the  block  due  to  not  tamp- 
ing the  concrete  sufficiently  during  construction  and  to 
the  use  of  poor  material;  and  to  the  crumbling  of  the 
top-dressing  or  grouting,  improperly  put  on  for  the  pur- 
pose of  leveling  it,  after  the  block  had  partly  set. 

An  instance  may  be  given  as  a  good  illustration  of 
concrete  mortar-block  troubles.  After  the  blocks  had 
partly  set,  they  were  grouted  up  1  in.  A  short  time  after 
the  mill  began  operating,  the  grouting  began  to  crumble, 
resulting  in  the  battery-posts  dancing  and  the  mortars 
shifting.  Then,  within  a  short  time,  occurred  breaking  of 
parts,  such  as  driving-pulley,  plates,  cam-shafts,  cams, 
stems,  and  mortar  anchor  bolts. 

Grouting  the  blocks  has  been  successful,  but  in  view 
of  the  large  number  of  cases  in  which  it  has  not,  it  is 


20  STAMP  MILLING  AND  AMALGAMATION 

inadvisable  to  take  such  chances.  It  is  necessary  to  make 
the  surface  of  the  block  absolutely  true,  though  it  may 
vary  a  small  fraction  of  an  inch  from  being  level;  this 
can  be  accomplished  by  chiseling  and  scraping  the  block 
after  it  has  partly  set,  or  by  promptly  bolting  a  wooden 
frame  to  the  top  of  the  block  by  means  of  the  anchor 
bolts,  to  press  it  into  shape,  before  it  has  had  time  to  set. 
Anchor  bolts  should  be  set  in  pipes  in  such  a  way  that 
should  one  break  it  can  be  unscrewed  and  a  new  one  in- 
serted. Iron  anvil-blocks  between  the  mortar  and  the 
concrete  are  superfluous  and  seldom  used.  The  concrete 
block  should  be  quite  wide,  while  the  mortar  base  should 
be  wider  and  heavier  than  is  usual  with  mortars  for 
wooden  blocks,  in  order  that  solidity  and  strength  may 
be  insured.  A  sheet  of  %-in.  rubber  is  placed  between 
the  mortar  and  the  block,  not  with  the  idea  of  cushioning 
the  force  of  the  stamp  blows,  but  to  make  an  even  bear- 
ing which  will  equalize  any  slight  irregularities  of  the 
concrete  surface  and  one  into  which  water  cannot  enter, 
for  a  combination  of  moisture  and  a  slight  jar  or  working 
of  the  mortar  would  tend  to  disintegrate  the  block.  There 
has  been  a  question  with  builders  as  to  whether  the  bat- 
tery-posts should  rest  in  foot  or  bed-plates  bolted  to  the 
concrete,  or  in  streak-sills  and  other  framing  as  with 
wooden  blocks.  With  the  posts  resting  on  timbers,  these 
timbers  will  absorb  and  minimize  the  jar  and  vibration 
to  some  extent,  thus  relieving  the  battery-posts  and  les- 
sening the  pounding  between  the  cam-shaft  and  its  boxes ; 
but  given  good,  wide  surfaces  in  both  block  and  mortar, 
with  the  mortar  and  the  battery-posts  bolted  securely  to 
the  block,  there  will  be  a  minimum  of  jar  and  vibration, 


FOOTING  OF  MORTAR  BLOCKS  21 

and  to  just  the  extent  that  this  is  reduced  will  the  break- 
ages and  the  wear  and  tear  be  lessened. 

Mortar-blocks  for  small  mills  are  usually  of  wood,  con- 
sisting of  pitch  pine  of  such  size  that  two  pieces  bolted 
together  make  a  block,  down  through  various  sizes  to 
ordinary  planking  spiked  into  a  solid  block  of  the  desired 
dimensions.  They  should  be  coated  with  a  preservative 
paint  to  lessen  the  tendency  to  rot.  In  length  they  vary 
from  8  to  25  ft.  Where  solid  rock  cannot  be  secured,  a 
bed  of  concrete  2  or  3  ft:  thick  and  as  wide  as  4  ft.,  is 
made  as  a  foundation.  Even  where  bedrock  is  found, 
a  bed  of  concrete  1  ft.  thick  is  advisable  to  give  a  level 
surface  for  the  block  to  rest  on,  and  to  fill  crevices  or 
weak  spots  in  the  rock.  After  the  block  is  set  in  place, 
sand  is  filled  around  it  and  tamped  down.  Pouring  in 
concrete  has  been  tried,  but  is  not  recommended  on  ac- 
count of  its  shrinking.  The  nuts  of  the  anchor  bolts,  tie 
rods,  and  all  others  about  the  battery  should  be  fre- 
quently tightened  after  the  mill  goes  into  operation. 


CHAPTER  II. 

Mortars  are  made  of  cast  iron  and  approximately  six 
or  more  times  heavier  than  the  weight  of  the  stamp  to 
be  used  in  them.  In  selecting  a  mortar  for  rapid  crush- 
ing it  is  advisable  to  get  a  narrow  one,  the  best  known 
type  being  the  'Homestake'.  All  manufacturers  make  a 
mortar  of  this  kind.  By  a  narrow,  rapid-crushing  mortar 
is  meant  one  having  as  little  spare  room  in  its  crushing 
area  as  "possible.  Such  a  mortar  is  usually  about  12  in. 
wide  at  the  discharge-lip,  and  there  is  no  surplus  space 
at  the  ends.  It  may  be  urged  that  but  little  inside  amal- 
gamation can  be  done  with  such  a  mortar.  This  idea  is 
erroneous,  for  inside  amalgamation  ordinarily  varies  as 
the  height  of  the  discharge,  which  is  the  vertical  distance 
between  the  tops  of  the  dies  and  the  bottom  of  the  screen. 
A  chuck-block  plate,  and  in  some  cases  a  back-plate,  can 
be  used  in  these  mortars,  though  it  will  require  some  care 
and  extra  trouble.  Inside  amalgamation,  as  referring  to 
the  catching  of  a  large  part  of  the  gold  inside  the  mortar, 
is  going  out  of  use.  Capacity  is  being  called  for,  and 
the  tendency  toward  outside  amalgamation,  or  at  least 
toward  not  requiring  so  large  a  proportion  of  the  gold 
to  be  caught  inside  the  mortar  at  the  expense  of  capacity, 
is  increasing.  It  has  been  suggested  that  by  using  a 
wide  mortar  it  may  be  lined  and  thus  converted  into  a 
narrow  one  if  desired.  It  is  hard,  however,  to  get  special 
liners  made  that  will  not  give  trouble  by  coming  loose. 
It  would  be  almost  impossible  to  decrease  the  horizontal 
distance  between  the  die  and  the  screen.  However,  the 


LINERS  IN  MORTARS  23 

idea  has  some  merit  and  can  be  applied  to  wide  mortars 
now  in  use.  The  feed-mouths  of  mortars  should  be  wide, 
so  that  coarse  rock  may  be  fed  to  them  during  break- 
downs of  the  rock-crusher. 

Double-discharge  mortars,  with  a  screen  opening  in 
front  and  behind,  have  been  tried,  in  amalgamating,  but 
are  not  considered  successful.  The  back-screen  is  hard 
to  get  at,  and  to  hold  in  place  without  coming  loose. 
There  appears  to  be  so  much  motion  to  the  pulp  that  it 
does  not  settle  on  the  dies,  and  in  consequence  the  mor- 
tars frequently  fill  up,  especially  when  feeding  fine  mate- 
rial. So  much  water  is  required  that  the  outside  plates 
cannot  do  good  amalgamating  with  this  dilute  pulp.  The 
single-discharge  mortar  can  be  made  to  discharge  the 
pulp  about  as  fast  as  made  when  treating  ordinary  rock. 
In  view  of  this  and  of  the  inconvenience  in  working  with 
a  double-discharge,  these  mortars  are  usually  found  with 
the  backs  closed,  except  with  such  easily  disintegrated 
material  as  gravel,  where  the  problem  becomes  one  of 
screening  rather  than  crushing. 

Mortars  should  be  lined,  front,  back,  ends,  bottom,  and 
where  the  feed  strikes  the  mortar  at  the  bottom  of  the 
feed  slot.  The  front,  back,  and  end-liner  should  dove- 
tail ;  the  back-liner  may  or  may  not  be  finally  bolted  into 
place.  Attention  should  be  given  to  securing  liners  that 
will  not  come  loose.  A  bottom-liner  of  one  piece  is  usually 
a  nuisance ;  sand  and  pieces  of  iron  creep  under  the  ends, 
causing  the  liner  to  sag  in  the  middle,  when  a  general 
movement  begins  which  may  result  in  displacing  the  dies 
when  they  become  worn  down.  Two-piece  liners  act  in  a 
similar  way.  Individual  plates  or  'false  dies'  are  what  is 
required.  Old  dies  worn  smooth  answer  well.  An  inch 


24  STAMP  MILLING  AND  AMALGAMATION 

of  sand  should  be  placed  between  them  and  the  mortar 
below,  likewise  between  them  and  the  'true  dies'  above. 
This  has  been  condemned  on  the  ground  that  it  cushions 
and  consequently  lessens  the  efficiency  of  the  stamp  blows, 
but  its  use  will  reduce  the  chances  of  cracking  the  mortar 
or  the  dies.  The  dies  should  be  plumbed  exactly  under 
each  stamp,  and  should  fit  snugly  in  the  mortar,  with 
just  enough  space  intervening  that  they  may  be  pried  out 
without  too  much  trouble  from  locking  each  other.  Should 
there  be  any  surplus  room  at  the  ends  or  sides,  the  dies 
should  be  wedged  with  a  piece  of  iron  or  steel.  This 
wedging  is  likely  not  to  hold  when  the  dies  become  worn, 
consequently  steps  should  be  taken  to  secure  liners  that 
will  completely  take  up  this  surplus  space,  thus  firmly 
keeping  the  dies  themselves  in  place.  There  is  nothing 
more  annoying  than  a  mortar  so  large  that  the  dies  shift 
from  their  proper  position  under  the  stamps.  In  putting 
in  a  new  set  of  dies,  it  is  advised  to  pack  coarse  rock 
around  them  and  to  run  with  a  heavy  feed,  that  is,  with 
a  thick  bed  of  pulp  on  the  dies,  for  a  few  hours  until  the 
dies  have  been  solidly  cemented  in  place. 

If  it  is  expected  to  use  a  low  discharge  in  crushing,  a 
shallow  mortar  should  be  ordered.  Build  up  the  old  or 
partly  worn  dies  by  false  dies,  and  use  a  high  chuck-block 
when  starting  a  new  set,  decreasing  the  screen-height  an 
inch  at  a  time,  as  the  dies  wear  down,  by  chuck-blocks 
and  wooden  strips  of  various  thickness,  until  at  the  last 
lowering  of  the  screen,  just  before  discarding  the  worn- 
out  dies,  there  is  nothing  placed  underneath  the  screen- 
frame.  In  short,  keep  the  height  of  discharge  as  nearly 
constant  as  possible  by  lowering  the  screen  an  inch  at  a 
time,  as  the  dies  wear,  and,  if  possible,  do  not  move  the 


SHOES  AND  DIES  25 

dies  until  they  are  worn  out.  Where  the  dies  are  re- 
moved in  the  monthly  clean-up,  start  the  new  dies  with- 
out the  false,  ones,  and  insert  the  false  dies  at  the  next 
clean-up  after  these  new  dies  have  been  worn  down. 

Shoes  and  dies  are  made  of  iron  and  steel  of  several 
kinds,  going  under  such  names  as  cast  and  chilled  iron, 
hammered,  forged,  cast,  chilled,  chrome,  and  manganese 
steel,  and  semi-steel.  The  millman  will  decide  for  him- 
self when  choosing  from  these,  keeping  in  mind  the  local 
conditions  affecting  the  nature  of  the  ore  and  the  cost 
of  supplies.  It  is  usually  cheaper  to  use  steel  than  iron, 
despite  its  increased  cost,  on  account  of  the  smaller  con- 
sumption of  steel  per  ton  of  ore  crushed  and  the  less  time 
lost  in  renewals  and  setting  of  the  tappets.  The  life  of 
a  set  of  steel  shoes  and  dies  may  roughly  be  estimated  at 
four  months  or  less,  as  against  half  that  length  of  time 
for  iron,  though  the  life  of  steel-parts  has  shown  on  dif- 
ferent ores  such  wide  variations  as  from  2%  to  9  months, 
and  four  times  that  of  iron.  A  material  having  the  max- 
imum hardness  and  the  minimum  brittleness  is  desired. 
The  limit  of  hardness  is  passed  when  the  shoes  and  dies 
chip,  crack,  or  break.  The  remedy  in  this  case,  presuming 
that  the  feed  has  not  been  kept  too  low  or  the  drop  too 
long  for  the  nature  of  the  ore  fed,  is  to  use  a  softer  die ; 
if  the  trouble  continues,  keep  trying  a  softer  material 
for  the  die  until  the  proper  limit  is  reached.  It  may  be 
that  the  shoe  in  use  is  too  hard  and  brittle,  when  a 
softer  or  another  kind  should  be  tried,  but  not  one  as 
soft  as  the  die.  The  wear  is  greatest  on  the  shoe,  and  it 
should  have  the  hardest  metal.  With  a  die  softer  than 
the  shoe,  the  two  wearing  faces  adjust  themselves  well 
to  each  other.  The  variation  of  life  between  a  steel  and 


26  STAMP  MILLING  AND  AMALGAMATION 

iron  shoe  is  much  greater  than  between  a  steel  and  iron 
die.  For  these  various  reasons  many  mills  are  found  using 
steel  shoes  and  iron  dies  with  great  satisfaction.  The 
use  of  a  steel  shoe  with  a  semi-steel  die  is  also  recom- 
mended. 

Should  the  die  be  found  to  wear  unevenly  on  one  side, 
it  may  be  due  to  a  soft  spot  in  the  metal,  which,  once 
started,  increases,  or  it  may  be  due  to  some  trouble  in 
the  feeding  of  the  mortar.  The  die  should  be  turned 
half-way  round  in  the  effort  to  make  it  wear  evenly. 
At  some  mills  the  dies  removed  when  cleaning  up  the 
mortars  are  returned  to  the  exact  place  and  position,  at 
others  they  are  turned  half-way  around;  in  other  mills 
no  attempt  is  made  to  return  them  to  any  particular 
position.  If,  on  examination  before  removing,  the  dies 
appear  to  be  wearing  evenly,  and  do  not  exhibit  a  tend- 
ency to  wear  unevenly  in  some  general  direction,  it  is 
unnecessary  to  use  care  to  return  them  to  any  particular 
position,  as  a  shoe  and  die  soon  adjust  themselves  to 
each  other.  The  shoe  generally  wears  to  an  even,  slightly 
concave  face,  while  the  die  wears  convex  in  a  correspond- 
ing manner,  this  is  called  'cupping';  the  parts  are  said 
to  'cup'.  The  evenness  of  this  cupping  is  due  to  the 
rotation- of  the  stamps  in  falling,  to  the  dies  being  plumbed 
exactly  underneath  the  shoes,  and  to  closely  fitting  guides 
which  enable  the  shoe  to  strike  the  die  exactly  central  at 
each  drop. 

No  die  should  be  permitted  to  stand  higher  or  lower 
than  the  others,  or  it  will  cause  that  stamp  to  'pound' 
or  to  'cushion'  through  having  too  thin  or  too  thick  a 
bed  of  pulp  over  its  corresponding  die.  In  the  effort  to 
economize  by  saving  'steel'  (the  general  term  for  shoes 


SETTING  SHOES  AND  DIES  27 

and  dies),  the  die  is  usually  worn  to  a  thickness  of  1 
or  1%  in- ;  at  less  than  this  thickness  it  is  liable  to  break 
at  any  time.  When  a  die  breaks  in  a  set  that  is  only 
partly  worn  out,  another  of  the  same  height  must  be  put 
in  its  place.  If  no  die  of  this  height  is  available,  a  new 
set  should  be  put  in,  though  it  is  possible  to  replace  the 
broken  die  with  a  shorter  one  built  up  by  a  false  or  old 
die.  All  mills  have  on  hand  an  assortment  of  partly 
worn  shoes  and  dies  of  various  heights  for  this  purpose. 
The  die  should  be  of  exactly  the  same  diameter  as  the  shoe 
or  14  in-  larger,  as  the  shoe,  through  the  looseness  of 
the  guides,  strikes  over  an  area  slightly  larger  than  its 
face. 

For  securing  shoes  in  the  boss-head,  hardwood  wedges 
made  from  staves  of  nail  kegs  or  barrels  are  tied  around 
the  shanks  of  the  shoes.  Soft  wood  can  be  used  if  it  is 
of  a  tough,  pliable  nature,  but  the  shoes  come  off  so 
easily  that  its  use  is  not  advised.  Thick  wedges  can  be 
used  where  the  space  calls  for  thin  ones,  by  spacing  them 
some  distance  apart  about  the  shank,  and  allowing  them 
to  be  crushed  into  shape  when  the  shoe  is  put  on.  With 
the  old  style  of  iron  bosses,  having  a  ring  on  the  lower 
end,  the  shoulder  of  the  shoe  should  not  come  in  contact 
with  the  boss  or  it  will  tend  to  loosen  the  ring.  In  bosses 
without  these  rings,  the  shoe  should  be  wedged  so  that 
it  will  be  driven  up  flush  with  the  boss,  for  the  shoe  can 
then  be  worn  to  almost  the  thinness  of  cardboard  before 
breaking.  For  removing  the  worn-out  shoes,  a  *  drift'  of 
tough  steel  is  inserted  in  the  key-way  in  the  centre  of 
the  boss-head  and  pounded  in,  the  wedging  tension  caus- 
ing the  shoe-shank  to  be  forced  out  of  the  socket.  Where 
the  shank  does  not  extend  into  the  field  of  the  key-way, 


28  STAMP  MILLING  AND  AMALGAMATION 

a  'dutchman,'  that  is,  a  piece  of  metal  2  in.  long,  usually 
a  broken  tappet-key,  is  slipped  in  on  top  of  the  shank, 
and  the  'drift'  is  carefully  inserted  to  make  a  tight  fit 
before  driving.  The  edges  of  the  'drift'  should  be  greased 
to  make  it  drive  easier.  Shoes  are  also  removed  by  blow- 
ing them  out  with  small  charges  of  dynamite.  Should 
the  ring  of  wooden  wedges  remain  'frozen'  to  the  socket 
without  dropping  out  with  the  shank,  they  should  be 
allowed  to  remain,  and  in  putting  on  a  new  shoe,  a 
square  piece  of  canvas  is  laid  on  the  shank  or  a  few  new 
thin  wedges  tied  about  it. 

In  putting  on  a  new  set  of  shoes,  the  stems  are  first 
cleaned  of  the  grease  below  the  tappets  by  passing  a  long 
strip  of  burlap  or  cloth  wet  with  kerosene  or  gasoline 
about  the  stem  and  alternately  pulling  each  end  of  the 
cloth  until  the  stem  is  clean.  This  is  necessary  as  the 
tappet  will  now  be  set  in  the  cleaned  part — it  would  be 
impossible  to  make  two  slippery,  greasy  metal  surfaces 
hold  together.  The  tappet-keys  are  loosened  and  the 
stem  pulled  up  through  the  tappet  by  means  of  the  over- 
head chain-blocks  until  the  boss  is  raised  sufficiently  to 
allow  a  new  shoe  to  be  set  underneath,  when  the  tappet 
keys  are  driven  in  sufficiently  tight  for  security,  and  the 
chain-blocks  removed.  The  new  shoes  are  now  rolled 
on  a  plank  up  to  the  mouth  of  the  mortar ;  they  are  stood 
on  the  mortar  lip  and  the  wooden  wedges  tied  about  the 
shanks ;  then  they  are  slipped  in  upon  the  dies  underneath 
the  bosses.  The  stem  is  now  dropped,  using  a  thick  cam- 
stick  to  increase  the  height  of  drop.  As  the  stem  drops, 
the  millman  places  his  hand  on  the  tappet  that  he  may 
be  able  to  tell  by  the  jar  if  the  shoe  has  been  picked  up 
by  the  boss.  He  keeps  the  cam-stick  between  the  tappet 


ADJUSTING  THE  DROP  29 

and  the  earn  at  each  lift  of  the  stamp,  so  as  to  give  the 
stamp  a  higher  drop,  and  consequently  a  greater  driving 
effect.  It  also  causes  the  stamp  to  revolve  more,  insuring 
a  straighter  driving  of  the  shoe  shank  into  the  boss.  As 
the  shoe  is  lifted  the  first  time,  the  man  at  the  mortar 
below  throws  underneath  it  a  shovelful  of  fine  rock  or 
pulp  to  cushion  the  blow  and  prevent  cracking  or  chip- 
ping of  the  shoe  or  die.  The  operation  is  spoken  of  as 
'driving  on  shoes',  and  is  followed  by  setting  each  stamp 
to  drop  the  exact  height  desired.  Some  millmen,  instead 
of  driving  the  tappet-keys  in  lightly  the  first  time,  set 
the  tappets  permanently  and  do  not  re-set  them  after  the 
shoes  are  on.  This  results  in  the  height  of  drop  being 
a  little  irregular,  but  some  men  are  able  to  calculate  so 
closely  just  how  far  the  shoe  will  be  driven  into  the  boss 
that  the  plan  is  often  a  good  one.  When  tying  wedges 
around  a  shoe  resting  under  a  boss,  it  should  be  made  a 
habit  to  pull  the  shoe  out  sufficiently  so  that,  should  the 
stamp  accidentally  or  otherwise  drop  off  the  finger-jack, 
it  will  fall  on  top  of  the  shank  instead  of  encircling  it, 
as  many  men  have  had  fingers  cut  off  in  this  way.  A 
safer  and  more  expeditious  method  is  to  tack  the 
wooden  wedges  to  a  strip  of  drilling  and  tie  them  about 
the  shank  of  the  shoe  before  placing  the  shoe  beneath  the 
boss. 

Should  a  shoe  come  off,  and  the  boss  continue  to  drop 
encircling  the  shank,  the  socket  of  the  boss  will  soon  be 
worn  so  large  that  a  shoe  cannot  again  be  fastened  in  it. 
Should  the  shoe  turn  partly  or  completely  over  on  its 
side,  the  shoulder  of  the  boss  may  be  so  battered  that  it 
may  be  necessary  to  remove  it  and  chip  out  the  socket. 


30  STAMP  MILLING  AND  AMALGAMATION 

Instead  of  doing  this,  the  boss  should  be  set  to  drop  en- 
circling the  shoe-shank  with  a  height  of  2  or  3  in.,  after 
which  the  entire  battery  should  be  run  for  a  half-hour, 
when  the  socket  should  be  worn  to  its  normal  size. 

The  boss-head,  or  'boss'  for  short,  should  be  of  cast 
steel  instead  of  iron,  as  steel  is  less  liable  to  be  split  by 
the  wedging  tendency  of  the  tapered  stem  or  shoe-shank, 
or  by  the  blowing  out  of  shanks  or  broken  stem-ends  with 
dynamite.  An  additional  reason  is  that  there  is  much 
wear  on  the  lower  part  of  the  boss  by  the  attrition  of  the 
pulp  when  the  shoes  are  well  worn  down,  as  can  be  seen 
by  examining  the  bosses  that  have  been  in  use  for  a  long 
time.  Steel  is  better  able  to  resist  this  abrasion,  as  is 
shown  by  the  comparative  life  of  iron  and  steel  shoes  and 
dies. 

Tappets  should  be  of  cast  steel,  being  more  durable 
than  cast  iron.  They  are  counter-bored  at  each  end  to 
have  a  recess  of  %  to  %  in.  wide  and  y2  in.  or  more  deep, 
so  that  the  entire  face  of  the  tappet  exposed  may  come 
in  contact  with  the  cam  which  it  rides,  and  thus  wear 
evenly.  As  the  cam  must  be  placed  a  fraction  of  an  inch 
away  from  the  stem,  if  the  tappet  were  not  counter- 
bored  in  this  way  a  thin  collar  of  metal  would  gradually 
form  about  the  stem  as  the  tappet-face  wore  down.  This 
collar  would  interfere  with  the  action  of  the  cam  on  the 
tappet. 

The  tappet  should  have  a  slight  counter-bore  through- 
out the  main  bore  through  which  the  stem  passes,  and  on 
the  side  opposite  the  gib  or  small  piece  of  steel  enclosed 
in  the  tappet  which  is  wedged  against  the  stem,  by  driv- 
ing in  the  tappet-keys.  This  counter-bore  should  be  of 


SETTING  TAPPETS  31 

a  smaller  radius  than  the  main  bore  of  the  tappet,  and 
should  make  the  main  bore  elliptical  in  form  by  reaming 
out  one-third  of  its  circumference.  When  the  tappet  is 
secured  to  the  stem  by  driving  the  keys  against  the  gib, 
it  forces  the  stem  into  the  elliptical  part  of  the  bore  and 
gives  three  bearing  surfaces  equidistant,  two  where  the 
counter-bore  intersects  the  main  bore,  and  one  at  the  gib, 
whereas  there  are  only  two  in  a  tappet  not  counter-bored. 
The  use  of  three  bearing  surfaces  instead  of  two,  with 
the  increased  wedging  effect,  enables  the  tappet  to  be 
firmly  fastened  without  driving  the  keys  excessively  tight. 
Slipping  of  tappets  is  one  of  the  banes  of  a  millman's 
existence,  and  advantage  should  be  taken  of  every  aid 
to  prevent  it.  All  manufacturers  do  not  counter-bore 
the  tappets  in  this  manner,  and  this  point  should  always 
be  taken  up  when  purchasing  a  mill  or  ordering  new 
tappets. 

Gibs  that  are  too  soft  tend  to  cut  out  at  the  key-ways 
by  the  frequent  driving  of  the  keys.  Placing  a  thick 
metal  shim  between  gibs  and  keys  will  prevent  this 
though  it  is  much  more  difficult  to  keep  tappets  from 
slipping  when  shims  are  placed  between  the  gib  and 
the  keys  than  when  placed  between  the  keys  and  the 
tappet  as  is  usually  done.  The  use  of  a  softer  metal  in 
the  key,  such  as  soft  steel  or  iron,  will  lessen  the  cutting 
tendency.  Gibs  that  are  cut  badly  should  be  removed. 
Broken  gibs  should  also  be  replaced  or  they  will  mar 
and  scratch  the  stems.  Tappet-keys  are  re-shaped  and 
re-pointed  at  the  blacksmith-shop,  or  by  means  of  a  coarse 
file.  An  emery  wheel  is  an  excellent  tool  for  this  pur- 
pose, and  should  be  included  in  the  equipment  of  every 


32  STAMP  MILLING  AND  AMALGAMATION 

mill.  For  driving  tappet-keys,  single-hand  hammers  with 
handles  a  little  longer  than  usual,  so  that  they  may  be 
used  when  necessary  with  two  hands  also,  should  be  em- 
ployed. When  tappet-keys  have  to  be  sledged  with  heavy 
double-hand  hammers  to  prevent  the  tappet  from  slipping, 
something  is  wrong;  usually  the  gibs  are  cut  out  at  the 
key-way  and  the  tappets  are  bored  so  large  in  comparison 
to  the  diameter  of  the  stem  that  there  is  little  binding 
surface.  Gibs  with  their  curves  bored  to  a  radius  slightly 
smaller  than  that  of  the  stem  exert  a  good  clamping  effect. 
Tappets  should  be  bored  1/32  in.  larger  in  diameter 
than  the  stems.  It  should  be  possible  to  move  and  slide 
them  easily,  but  no  looseness  should  be  apparent.  Tappets 
with  a  bore  of  Ve*  *n-  larger  than  the  stem  have  been 
used,  but  too  much  trouble  is  experienced  in  slipping  them 
over  irregularities  in  the  stem.  The  driving  of  the  keys 
should  be  done  evenly.  If  the  upper  or  lower  key  be 
driven  tight  before  starting  the  other,  it  may,  in  the 
case  of  tappets  bored  too  large,  throw  the  tappet  out  of 
line  with  reference  to  the  stem.  This  may  be  one  of  the 
causes  of  stamps  twirling.  It  may  also  serve  to  explain 
the  lengthening  of  the  drop  of  a  stamp  by  a  slipping  tap- 
pet, which,  however,  is  rarely  seen.  Ordinarily,  when  a 
tappet  slips,  it  is  driven  upward  on  the  stem  by  the  re- 
peated blows  of  the  cam,  thus  lowering  the  stamp  until 
the  shoe  touches  the  die,  after  which  there  can  be  no 
further  lifting  of  the  stamp.  Taking  the  case  where  the 
upper  key,  or  the  upper  part  of  a  broken  gib,  is  driven 
tight  first  and  the  tappet  is  slipping,  the  gib  will  be  out 
of  alignment  with  the  stem  and  the  upper  part  of  the 
gib  pressed  tightly  against  the  stem  while  the  lower  is 


DRIVING  TAPPET  KEYS  33 

inclined  from  it.  It  is  impossible  for  the  tappet  to  be 
driven  upward  as  usual,  for  the  upper  shoulder  of  the  gib 
buries  itself  in  the  stem  and  prevents  such  movement, 
but  the  jar  and  rebound  from  the  stamp  striking  the  die 
causes  the  tappet  to  slip  down  on  the  stem  a  little,  where 
it  takes  a  new  grip  the  moment  the  cam  strikes  it,  causing 
it  to  drop  down  a  little  farther  when  the  stamp  strikes 
again.  This  continues  until  the  stamp  begins  to  'cam', 
that  is,  to  fall  on  the  cam  from  having  too  long  a  drop 
for  the  speed  of  revolution. 

If  the  tappet-keys  on  each  battery  are  all  driven  in  on 
the  same  side,  that  is,  all  driven  in  either  on  the  right  or 
the  left  side  of  their  respective  stems,  there  will  be  less 
trouble  from  the  locking  or  meshing  of  keys  that  project 
out  too  far,  or  are  slipping  out  of  the  key-ways.  Tap- 
pets are  often  made  with  the  key-ways  wider  on  one 
side  than  on  the  other,  so  that  the  keys  shall  be  driven 
in  from  the  wide  side.  This  is  imperative  only  when 
the  gibs  are  badly  worn,  but  in  such  cases  the  tappet 
should  be  marked  by  chalk  or  paint  to  denote  this  side, 
unless  the  manufacturers  have  marked  it. 

For  setting  the  tappets,  a  special  quick-acting  clamp 
is  placed  around  the  stem  at  the  required  distance  above 
the  tappet,  which  is  then  resting  on  the  finger-jack,  and 
on  loosening  the  keys  the  stem  drops  down  this  distance ; 
the  clamp  is  then  removed  after  tightening  the  keys. 
Otherwise  the  chain-blocks  are  attached  to  the  stem  to 
raise,  lower,  or  hold  it.  In  setting  the  tappets,  a  man 
stands  on  each  of  the  two  opposite  sides;  one  places  a 
'drift'  having  an  eye  and  a  wooden  handle  against  the 
end  of  the  key  and  loosens  it  with  a  hammer,  but  does 


34  STAMP  MILLING  AND  AMALGAMATION 

not  drive  it  out,  while  the  man  on  the  other  side  holds 
the  tappet  steady  and,  with  a  piece  of  cloth  to  deaden 
the  force  of  the  blows,  holds  the  key  from  being  suddenly 
hurtled  out  of  the  key-way  by  the  blow  of  the  hammer. 
After  the  stem  has  been  adjusted  in  the  tappet,  this 
second  man  hammers  the  keys  back  into  place.  Where 
the  tappets  are  bored  to  a  close  fit  on  the  stems,  it  is 
customary  for  one  man  to  loosen  the  keys  slightly,  while 
the  other  makes  a  quick  drive  at  one  of  the  keys  when 
the  tappet  has  slipped  to  the  mark;  in  this  way  the  tap- 
pet setting  proceeds  rapidly.  A  chalk  mark  is  placed 
around  the  stem  above  the  tappet,  so  that  any  slipping 
may  be  readily  noticed.  If  the  tappets  stick  and  refuse 
to  move,  a  little  kerosene  or  gasoline  may  be  poured  into 
the  counter-bore  at  the  top  to  run  down  between  the 
stem  and  the  tappet,  while  the  tappet  is  struck  with  a 
hammer  on  the  inside  of  the  collar  or  on  the  waist.  The 
outside  or  wearing  face  must  never  be  roughened  by 
being  struck.  The  cam-stick  may  be  used  if  desired  so 
that  the  blow  of  the  cam  may  be  utilized  to  drive  the 
tappet  upward  on  the  stem  without  dropping  the  stamp 
off  the  finger-jack.  When  setting  tappets,  it  is  customary 
to  hang  up  only  the  stamp  that  is  being  set,  allowing  the 
other  four  stamps  to  continue  dropping,  with  the  possible 
exception  of  when  setting  the  stamp  which  actuates  the 
feeder.  To  lessen  the  danger  to  fingers,  two  stamps  may 
be  hung  up,  the  one  being  set,  and  its  neighbor  on  the 
side  next  to  the  keys. 

Cams  should  be  of  steel  to  insure  long  life  and  to  lessen 
the  chances  of  being  broken,  an  undesirable  occurrence 
on  account  of  the  labor  involved  in  replacing  with  a  new 


RIGHT  AND  LEFT-HAND  CAMS  35 

cam.  They  are  set  from  %  to  *4  in.  away  from  the 
stems.  Cams  are  called  'right-hand'  and  'left-hand',  and 
are  determined  by  the  following  rule:  when  the  hub  of 
the  cam  is  toward  the  observer  and  the  cam  rotates  to 
the  right,  or  clockwise,  it  is  a  'right-hand'  cam;  if  the 
rotation  is  to  the  left,  or  counter  clockwise,  it  is  a  'left- 
hand'  cam.  The  action  of  the  cams  upon  the  tappets 
tends  to  cause  the  cams  to  move  away  from  the  stamps. 
This  lateral  thrust  of  the  cams  and  cam-shaft  is  greatest 
at  the  moment  when  each  cam  leaves  its  tappet.  It  is 
overcome  in  the  10-stamp  cam-shaft  by  making  the  cams 
of  one  battery  left-hand,  and  those  of  the  other  right- 
hand,  the  lateral  thrust  of  one  set  in  one  direction  over- 
coming that  of  the  other  set  in  the  other  direction.  With 
a  five-stamp  cam-shaft  it  is  possible  to  use  two  right-hand 
and  three  left-hand  cams,  but  generally  the  boss  of  the 
cam-shaft  pulley  is  utilized  as  a  collar  in  connection  with 
other  collars  to  prevent  the  lateral  movement  of  the  shaft. 
It  is  highly  important  that  the  cams  be  properly  de- 
signed for  the  speed  and  length  of  drop  to  be  used.  The 
design  should  be  such  that  when  the  shoe  strikes  the  die 
of  an  empty  mortar  the  space  of  ^4  in-  °r  more  will  in- 
tervene between  the  faces  of  the  tappet  and  the  cam,  near 
the  hub  of  the  latter,  and  that  the  cam  will  immediately 
engage  the  tappet  at  a  low  but  rapidly  increasing  speed 
as  the  tappet  is  lifted  to  the  point  where  it  drops  off  the 
point  of  the  cam.  One  of  the  greatest  sources  of  noise, 
wear  and  tear,  and  broken  parts,  arises  from  a  cam  im- 
properly designed  and  which  suddenly  engages  or  vio- 
lently strikes  the  tappet. 


CHAPTER  III. 

Stems  were  formerly  made  of  wrought  iron,  but  now 
almost  always,  if  not  in  every  case,  of  mild  steel.  Stems 
do  not  wear  out  directly,  but  indirectly  by  -breaking. 
These  breakages  occur  mainly  at  the  point  where  the  stem 
leaves  the  embrace  of  the  boss,  but  occasionally  an  old 
stem  breaks  near  the  tappet.  Breakages  at  other  points 
so  seldom  occur  as  to  be  phenomenal.  When  the  stem 
breaks  at  the  boss  it  is  reversed  and  the  other  end  is 
placed  in  the  socket  of  the  boss.  When  this  second  end 
breaks,  the  stem  is  sent  to  the  blacksmith-shop  to  be 
swaged  down  again  at  each  end,  or  to  the  lathe  to  be 
turned  down,  after  which  it  is  returned  to  the  mill  to 
be  used  again.  After  SQ  many  breakages  have  occurred 
that  the  stem  will  no  longer  run  in  the  guides  from  being 
too  short,  a  new  piece  is  wrelded  to  it,  or  in  the  absence 
of  facilities  for  making  such  a  weld  the  stem  is  discarded. 

After  welding,  if  bent,  a  stem  may  be  straightened  by 
laying  it,  while  still  hot,  in  the  groove  between  two  old 
stems  or  heavy  pipes,  of  a  diameter  similar  to  the  stem, 
placed  parallel  and  an  inch  apart  on  the  ground  where 
they  are  firmly  held  by  straps  or  bolts.  In  this  groove 
the  stem  may  be  hammered  and  straightened. 

The  breaking  of  stamp-stems  and  cam-shafts  has  been 
popularly  attributed  to  the  crystallization  of  the  metal, 
the  theory  being  that  the  continuous  jar  and  vibration 
to  which  these  parts  are  subjected  causes  a  molecular 
change  whereby  the  fibrous  structure  of  the  metal  is 
changed  to  a  granular,  crystalline  form,  which,  not  hav- 


CRYSTALLIZATION  OP  STAMP  STEMS  37 

ing  the  tenacity  of  the  fibrous  structure,  finally  breaks. 
Many  arguments  have  been  advanced  for  and  against  this 
theory,  but  nothing  conclusive  has  been  shown.  It  is 
pointed  out  that  the  break  shows  an  apparently  crystal- 
line structure  because  it  is  across  the  grain  of  the  metal 
in  the  stem,  and  that  there  is  no  evidence  to  show  that 
any  change  takes  place  in  the  structure,  either  in  the 
case  of  a  broken  stamp-stem  or  in  any  experiment  made 
in  the  general  way.  It  is  also  pointed  out  that  should 
crystallization  take  place,  the  breaks  would  not  be  con- 
fined so  generally  to  one  particular  point.  The  argument 
has  been  presented  that  the  jar  and  vibration  from  the 
impact  of  the  shoe  upon  the  die  passes  into  the  boss,  and 
that  this  vibratory  motion  concentrates  to  pass  into  the 
small  cross-section  of  the  stem  at  the  point  where  the 
stem  leaves  the  boss,  and  here  is  the  point  of  greatest 
stress.  Under  the  strain  the  particles  of  metal  lose  their 
power  of  cohesion;  the  metal  develops  'fatigue7;  minute 
fractures  occur;  there  is  a  repeated  bending  stress  in 
different  directions  from  the  stamp  striking  on  an  uneven 
surface,  as  when  a  piece  of  coarse  rock  is  lying  on  the 
edge  of  the  die ;  and  eventually  the  minute  fractures  de- 
velop into  a  break.  Conditions  prevail  in  the  stem  at 
the  tappet,  where  the  breaks  sometimes  occur,  somewhat 
similar  to  those  at  the  boss.  There  is  a  bending  strain 
from  the  cam  striking  the  side  of  the  tappet  and  away 
from  the  centre  of  the  stem.  As  the  tappet  is  continually 
shifted  up  and  down  the  stem  and  its  embrace  is  such 
that  the  vibratory  motions  are  not  communicated  to  the 
stem  so  constantly  at  one  point  as  at  the  boss,  the  strain 
there  is  less  and  breakages  do  not  so  frequently  occur. 


38  STAMP  MILLING  AND  AMALGAMATION 

The  practical  millman  will  ask:  "If  the  breakages 
are  not  due  to  the  crystallization  of  the  metal,  to  'what 
are  they  due?"  In  the  absence  of  any  authoritative 
reply  or  positive  proof  to  the  contrary,  it  is  as  well  to  con- 
tinue to  speak  of  them  as  being  due  to  that  cause. 

The  breaking  of  stems  can  be  prevented  by  annealing 
them,  that  is,  by  heating  and  slow  cooling.  This  fact 
gives  color  to  the  theory  that  crystallization  or  some 
change  in  the  structure  of  the  metal  does  take  place. 
Annealing  is  not  feasible  except  when  welding  or  re- 
pointing  the  stems.  The  breakages  have  been  partly  pre- 
vented by  boring  the  bosses  and  making  the  ends  of  the 
stems  larger.  It  is  a  question  among  millmen  whether 
steel  stems  break  less  frequently  than  those  made  of  iron. 
In  general,  they  do  break  less,  and  experiments  show 
that  mild  steel  will  stand  a  much  greater  strain  than 
wrought  iron  before  breaking,  yet  in  some  mills  where 
both  iron  and  steel  stems  are  in  use,  the  iron  has  been 
found  superior.  This  is  because  the  steel  in  the  stems 
is  of  a  poor  quality. 

With  a  view  to  reducing  the  bending  strain  and  caus- 
ing less  wrenching  of  the  stem  when  striking  an  uneven 
surface,  stamps  have  been  designed  in  which  the  centre 
of  gravity  is  placed  as  low  as  possible.  Both  guides  are 
bored  to  a  tight  fit  on  the  stem,  with  the  lower  guide 
placed  near  the  boss.  The  stem  is  made  short,  using  a 
long  boss  to  make  up  the  required  weight,  the  result  being 
that  the  stem  drops  straight  and  true,  and  that  there  is 
a  minimum  of  bending  and  wrenching  when  the  shoe 
strikes  away  from  the  centre  line  of  the  stamp.  The  re- 
sults with  these  stamps  appear  to  show  the  correctness 
of  the  theory  upon  which  they  are  built. 


SETTING  THE  BOSS  39 

Running  the  stamps  with  the 'feed  too  low  or  the  mor- 
tar empty — 'pounding  steel' — shortens  the  life  of  the 
stems.  When  a  piece  of  vagrant  steel  lodges  on  the  top 
of  a  die,  as  is  usually  the  case  when  a  stamp  suddenly 
begins  to  drop  harder  and  shorter  than  its  neighbors  in 
the  same  battery,  it  should  be  removed  or  it  may  cause 
the  stamp  to  break.  A  die  tipping  over  in  the  mortar 
will  cause  the  stamp  to  act  in  the  same  way  and  will  be 
productive  of  the  same  evil  result.  Concrete  mortar- 
blocks  were  formerly  supposed  to  be  more  severe  on 
stems  than  wooden  blocks,  but  the  results  in  good  in- 
stallations, where  everything  is  kept  solid  and  tight,  dis- 
prove this. 

In  putting  stems  into  bosses,  the  boss,  with  or  without 
the  shoe,  should  rest  on  the  die  and  not  on  a  thick  bed 
of  pulp.  The  boss  should  be  placed  exactly  underneath 
the  stem,  which  should  be  raised  just  sufficiently  to  do 
this  or  to  give  the  right  height  of  drop  after  the  parts  are 
fastened.  Two  strips  of  canvas,  2  in.  wide  by  15  in.  or  more 
long,  should  be  placed  across  the  socket  of  the  boss  at 
right  angles  to  each  other  and  slightly  pushed  in.  The 
stem  is  now  dropped  off  the  finger-jack  into  the  socket 
of  the  boss  by  means  of  the  cam-stick,  and  is  allowed  to 
be  dropped  by  the  revolving  cams  until  the  boss  is  caught 
and  the  stem  is  driven  in.  The  stamp  should  be  rotated 
while  dropping  so  that  the  stem  may  be  driven  straighter 
into  the  boss.  Care  must  be  exercised  to  have  the  boss 
placed  properly  and  to  see  that  the  guides  are  tight,  so 
that  the  stem  may  drop  directly  into  the  socket  instead 
of  striking  on  top  of  the  boss.  A  chalk  mark  is  placed 
near  the  end  of  the  stem  to  indicate  just  how  far  it  can 
be  allowed  to  enter  the  boss,  since  should  it  enter  too 


40  STAMP  MILLING  AND  AMALGAMATION 

far,  it  would  be  impossible  to  insert  the  'drift'  for  ^driv- 
ing out  the  'plug'  or  broken  stem-end,  should  the  stem 
break  again.  The  stem  may  be  lowered  into  the  socket, 
and  should  it  appear  that  it  will  enter  too  far,  more  strips 
of  canvas  should  be  used.  It  is  a  practice  with  many 
millmen  to  lower  the  stem  into  the  socket  and  drive  it 
in  by  pounding  with  a  heavy  hammer  on  the  upper  end. 
This  should  never  be  permitted,  as  it  is  certain  to  spoil 
the  tapered  end  so  that  it  will  not  fit  well  in  the  socket 
when  reversed,  or  should  it  be  a  broken  end,  it  will 
'mushroom'  into  a  jagged  end  over  which  a  tappet  can- 
not be  slipped. 

When  a  stem  pulls  out  of  a  boss  and  runs  for  some 
time  before  being  seen  and  hung  up,  the  tapered  end  is 
usually  pounded  out  of  shape  to  fit  the  socket,  though 
it  may  not  be  apparent  to  the  eye.  If  the  stem  will  not 
catch  again,  it  should  be  hoisted  out  of  the  mortar  and 
allowed  to  rest  on  a  plank  across  the  top  of  the  mortar, 
where  it  can  be  chipped  and  dressed  with  cold  chisels 
and  files.  It  is  reported  that  some  millmen  do  not  turn 
their  broken  stems,  but  that  two  men  dress  down  the 
broken  end,  using  cold  chisels  with  wooden  handles  and 
double-hand  hammers ;  it  is  doubtful  if  a  satisfactory  job 
could  be  done,  unless  the  required  taper  is  slight. 

Canvas  should  always  be  placed  in  the  sockets  as  it 
will  lessen  the  number  of  breakages.  When  steel  is 
wedged  tightly  against  steel,  it  becomes  as  if  made  of 
one  piece  and  all  the  jar  and  vibratory  motion  is  com- 
municated to  the  stem  at  the  point  where  it  leaves  the 
embrace  of  the  boss.  By  placing  canvas  between  the 
parts,  the  tendency  is  for  the  jar  and  vibration  to  be 
more  generally  distributed,  instead  of  concentrated  at 


SECURING  THE  STEM  IN  THE  BOSS  41 

one  point.  Likewise  the  distribution  of  the  bending-strain 
may  be  over  a  larger  area.  While  canvas  causes  the  parts 
to  hold  together  as  well  or  better  than  if  not  used,  it 
allows  the  stem  or  broken  end  to  be  removed  more  easily 
by  the  usual  means  of  drifting,  or  blowing  out  with  dyna- 
mite. Where  it  is  not  used,  the  parts  tend  to  rust  to- 
gether so  that  sometimes  a  boss  is  blown  to  pieces  in 
the  effort  to  remove  it  from  the  stem.  When  canvas  fails 
to  make  the  stem  stick,  a  shim  made  of  tough  metal 
should  be  tried,  such  as  a  thick  screen  plate,  or  thin 
sheet-iron  bent  cylindrically  and  set  in  the  socket;  or 
the  metal  may  be  cut  in  strips  and  shaped  to  fit  in  the 
socket  in  the  same  manner  as  canvas  strips.  If  the  stem 
still  fails  to  stick,  it  may  be  that  the  taper  does  not  fit 
the  socket  and  requires  dressing  down;  or  both  the  stem 
and  the  socket  may  be  too  smooth  to  catch  and  bind,  in 
which  case  a  small  stream  of  some  fine  grit,  such  as  jasper, 
should  be  allowed  to  run  into  the  socket  with  the  stem 
rising  and  falling  so  that  the  surfaces  may  be  roughened 
and  eventually  bind  on  one  another,  or  the  surfaces  may 
be  roughened  by  denting  with  chisels.  Some  sterna  will 
refuse  to  hold  in  the  bosses,  requiring  the  utmost  patience 
before  being  finally  fastened.  The  last  resort  is  to  turn 
the  stem  or  put  in  another  boss.  Stems  are  fastened, 
whenever  possible,  through  the  top  of  the  mortar  without 
stopping  the  other  stamps. 

When  a  stem  breaks,  the  first  thing  to  do  is  to  hang 
it  up  on  the  finger- jack,  allowing  the  others  to  run  until 
ready  to  change  this  stem;  should  this  not  be  for  some 
time,  the  boss  with  its  shoe  should  be  removed  from  the 
mortar.  When  ready  to  change  the  stem,  the  battery  is 
'pounded  out'  or  'stamped  out',  which  consists  in  shut- 


42  STAMP  MILLING  AND  AMALGAMATION 

ting  off  the  feed  and  allowing  the  stamps  to  run  as  Jong 
as  may  be  safe,  so  as  to  remove  as  much  pulp  as  possible 
from  the  mortar,  the  feed-water  being  shut  off  just  before 
hanging  up  the  stamps.  The  screen  is  then  removed,  to- 
gether with  the  chuck-block,  and  the  boss  is  inclined  out- 
ward from  the  mortar,  the  .*  drift '  inserted  in  the  key-way 
and  the  broken  end  driven  out,  after  which  the  boss  is 
righted  into  its  place.  Where  the  mortar-opening  is  too 
small  vertically  to  allow  the  boss  to  be  thus  righted  into 
its  place,  as  is  liable  to  be  the  case  with  a  boss  having 
a  new  shoe,  a  small  rope-block  is  attached  to  the  lower 
battery-girt  and  extended  down  through  the  top  of  the 
mortar,  and  the  boss  and  shoe  are  pulled  into  position, 
after  which  the  mortar  is  closed  and  the  other  stamps  are 
started  dropping.  Two  or  more  turns  of  a  stout  chain 
are  now  taken  about  the  tappet  of  the  broken  stem,  and 
the  chain-blocks  hooked  into  this  chain.  The  stem  is 
raised  until  the  tappet  nearly  touches  the  upper  battery- 
girt.  The  battery  is  now  hung  up  and  power  thrown  off 
from  the  cam-shaft.  Both  the  upper  and  lower  guides, 
which  have  been  previously  loosened,  are  now  removed, 
and  the  stem  and  tappet  are  swung  clear  of  the  battery- 
girt.  Raising  of  the  stem  is  continued  until  it  is  possible 
to  swing  the  lower  end  out  on  the  cam-floor.  If  it  is 
only  desired  to  reverse  the  stem,  it  can  now  be  done 
and  returned  to  its  place ;  but  should  it  be  desired  to  put 
in  a  new  stem,  one  usually  being  at  hand  for  such  cases, 
the  old  one  is  lowered  to  the  floor,  and  the  new  one  is 
picked  up  and  swung  into  place.  As  soon  as  the  stamp 
is  swung  into  position,  the  other  stamps  of  the  battery 
are  started  dropping.  The  guides  are  put  back  and  the 


DRIVING  OUT  THE  STEM  43 

tappet  is  temporarily  adjusted  for  fastening  the  stem  in 
the  boss,  as  explained  before. 

Some  men  are  able  to  turn  or  change  a  stem  without 
stopping  the  cam-shaft  and  the  other  stamps,  but  it  is 
so  dangerous  to  limb,  life,  and  machinery  that  it  should 
not  be  attempted.  If  the  plug  does  not  easily  drift  out 
of  the  boss,  the  boss  is  removed  to  a  more  convenient 
spot  for  driving  the  'drift',  or  a  new  boss,  together  with 
the  old  shoe,  is  used.  Blowing  the  plugs  out  with  dyna- 
mite is  a  lazy  man's  refuge  and  may  split  the  boss,  espe- 
cially when  made  of  iron.  At  one  mill  where  trouble 
has  been  experienced  from  the  plugs  not  readily  drifting 
out,  the  boss  is  heated  until  the  canvas  chars,  when  the 
plug  will  drift  out  easily. 

It  is  customary  to  allow  the  stamps  to  increase  their 
height  of  drop  through  the  wearing  away  of  the  shoe 
and  die,  from  i/2  to  1  in.  before  re-setting.  There  are 
several  ways  of  measuring  the  height  of  drop  and  the 
amount  it  must  be  decreased.  One  way  is  to  open  the 
mortar  and  measure  the  distance  between  each  shoe  and 
die.  This  is  an  impractical  way,  requiring  too  much 
labor,  while  the  measurements  are  unreliable  if  taken 
from  a  spot  in  the  die  that  has  cupped  unevenly,  or  if 
the  finger-jacks  are  of  an  uneven  height. 

Another  method  of  measuring  the  height  of  drop  is  to 
hold  a  piece  of  metal,  such  as  the  shims  used  with  tappet- 
keys,  against  the  stem  at  the  guide  and  measure  the 
scratch-marks  with  a  rule  while  the  stamp  is  running; 
the  measurements  obtained  in  this  way  are  also  liable  to 
be  inexact.  A  better  method  is  to  hang  up  each  stamp, 
rub  off  some  of  the  surplus  grease  above  the  upper  guide, 
and  oil  well  before  dropping  the  stamp.  After  all  the 


44  STAMP  MILLING  AND  AMALGAMATION 

stamps  in  a  battery  have  been  treated  in  this  way,  hang 
up  the  feed-stamp  or  shut  off  the  feed  as  long  as  safe. 
The  oil-marks  will  now  show  the  exact  relative  drop  of 
each  stamp  when  they  are  hung  up.  Should  the  finger- 
jacks  be  uneven,  the  oil-marks  should  be  measured  while 
the  stamps  are  running.  The  tappets  can  be  set  by  these 
oil-marks  and  re-checked  after  dropping  again. 

The  best  method  of  adjusting  the  height  of  drop  is  for 
the  millman  to  examine  the  stamps  once  daily,  and  by 
his  experienced  eye  single  out  those  stamps  that  are 
dropping  too  long  and  too  hard.  Laying  his  fingers  on 
the  tappets  or  stems,  he  feels  these  stamps  striking  harder 
than  their  neighbors  in  the  same  battery,  and  in  conse- 
quence he  reduces  their  drop  %  in.  He  aims  to  have 
the  individual  stamps  of  a  battery  strike  with  an  equal 
firmness,  so  that  all  may  have  an  equal  or  maximum 
crushing  effect,  and  so  that  he  may  be  able  to  feed  the 
battery  down  to  a  point  where  the  greatest  capacity  in 
crushing  effect  can  be  obtained.  He  also  aims  to  run  with 
the  maximum  of  drop  permissible  with  the  speed  set. 
These  are  some  of  the  secrets  of  getting  a  large  tonnage 
through  a  battery,  and  attention  to  them  may  result  in  in- 
creasing the  capacity  from  10  to  20%,  as  against  a  battery 
in  which  one  stamp  comes  down  hard,  while  its  neighbor 
is  cushioned,  or  where  the  drop  is  not  kept  as  long  as  pos- 
sible within  limits  of  safety. 

The  inside  of  the  mortar  should  be  occasionally  exam- 
ined to  ascertain  how  the  shoes  and  dies  are  wearing, 
and  also  to  look  for  any  fragments  of  broken  steel  that 
may  have  fallen  in,  or  have  come  in  with  the  ore.  This 
latter  should  always  be  attended  to  when  changing 
screens. 


ROTATION  OF  STAMPS  45 

The  friction  of  the  cam  against  the  tappet  causes  the 
stamp  to  rotate  while  running.  This  is  necessary  that 
the  face  of  the  tappet  may  be  worn  smoothly  all  around, 
and  also  that  the  shoes  and  dies  may  wear  or  cup  evenly. 
One  complete  rotation  of  the  stamp  is  made  in  from  5 
to  30  drops  of  the  present  style  of  stamps  under  normal 
conditions.  The  data  of  two  extreme  cases  will  give  some 
idea  of  the  cause  of  this  variation.  In  the  Gilpin  county 
practice  a  stamp  weighing  550  lb.,  dropping  17  in.  30 
times  per  minute,  rotates  1%  times  per  drop.  In  a  cer- 
tain mill  using  1500-lb.  stamps,  dropping  6  in.  and  110 
times  per  minute,  the  stamps  make  one  complete  revolu- 
tion in  30  drops.  The  stamps  in  two  mills  having  the 
same  weight  of  stamps  and  the  same  adjustments,  will 
vary  in  their  speed  of  rotation,  due  to  different  shaped 
cams  and  to  the  amount  of  lubricant  on  them.  Where  the 
stamps  rotate  too  fast,  there  is  a  small  loss  of  power 
and  too  much  wear  on  the  cams  and  tappets.  This  twirl- 
ing of  the  stamps  may  be  caused  by  the  wearing  parts 
being  devoid  of  grease,  by  being  roughened  through  run- 
ning without  grease,  or  by  the  .tappet  being  out  of  align- 
ment with  the  stem. 

Cam-shafts  are  sometimes  of  hammered  wrought  iron, 
but  usually  of  hammered  mild  steel.  A  material  is  re- 
quired for  stems  and  cam-shafts  that  is  tough  rather  than 
brittle,  and  is  able  to  withstand  the  tendency  to  crystal- 
lize and  break  under  impact;  a  good  grade  of  wrought 
iron  may  answer  these  requirements  as  well  as  steel,  and 
such  cam-shafts  are  recommended  from  experience.  Cam- 
shafts break  from  the  same  cause  as  stems,  from  'fatigue' 
and  crystallization  of  the  metal.  This  is  caused  by  the 
impact  of  the  cams  upon  the  tappets,  and  in  a  poorly 


46  STAMP  MILLING  AND  AMALGAMATION 

constructed  mill,  where  the  battery-posts  jump  and  vi- 
brate and  are  out  of  line,  by  pounding  in  the  bearing 
boxes.  The  breaking  of  a  cam-shaft  is  a  serious  thing 
as  compared  with  the  breaking  of  a  stem,  since  it  involves 
considerable  time  and  labor,  and  often  a  complete  loss 
of  the  cam-shaft.  It  was  customary  to  make  the  shafts 
for  the  lighter  stamps  from  4%  to  5  in.  diam.  As  the 
weight  of  stamps  has  increased,  the  diameter  of  the  cam- 
shafts has  not  always  correspondingly  increased,  and  this 
has  been  one  cause  of  shafts  breaking.  A  10-cam  shaft 
for  1000-lb.  stamps  should  be  6  in.  or  more  in  diameter, 
14  to  15  ft.  long,  and  will  weigh  upward  of  1400  Ib.  For 
1250-lb.  stamps  a  shaft  7  in.  diam.  and  weighing  1800  Ib. 
or  more  should  be  used.  Five-cam  shafts  do  not  require 
to  be  so  large  in  diameter  for  the  same  weight  of  stamps 
as  10-cam  shafts.  With  the  10-cam  shafts  now  in  use 
there  are  three*  bearings.  These  get  out  of  alignment  by 
the  shifting  of  the  battery-frame  and  the  great  wear  and 
tear  peculiar  to  a  stamp-battery,  which  tends  to  throw 
the  weight  and  stress  on  two  boxes  or  bearings,  making 
too  great  a  strain  for  a  long  shaft,  so  that,  as  the  shaft 
becomes  weakened  from  the  'fatigue'  or  crystallization 
of  the  metal,  it  is  liable  to  break,  especially  when  a  stamp 
is  'camming'  on  it  near  a  non-supporting  bearing.  The 
bearing-boxes  should  be  securely  bolted  to  the  battery- 
posts,  and  when  the  shaft  commences  to  throw  out  puffs 
of  air  from  the  boxes,  or  to  ' trash',  pound,  heat,  or  vi- 
brate in  them,  they  should  be  immediately  babbitted  that 
they  may  be  in  exact  alignment  and  have  three  good 
bearings.  If  cast-iron  boxes  without  babbitt  are  in  use,  the 
shaft  should  be  removed  and  these  boxes  aligned.  No 


THE  CAM-SHAFT  BOXES  47 

shimming  should  be  used  under  them  as  it  will  work  loose 
from  the  jar  about  a  battery. 

There  is  a  great  advantage  in  using  5-cam  shafts  in 
that  there  are  few  breakages.  As  there  are  but  two  bear- 
ings, even  if  the  boxes  do  get  out  of  line  there  is  no  ab- 
normal strain  at  any  time.  These  shafts  enable  one  bat- 
tery to  be  shut  down  to  change  a  stem  or  for  working 
on  the  interior  of  a  mortar  without  interfering  with  the 
operation  of  another.  The  disadvantages  are  a  slight 
increase  in  the  length  of  the  mill  and  the  power  required, 
also  a  doubling  of  the  number  of  battery  belts,  pulleys, 
and,  in  some  mills,  the  belt-tighteners.  There  is  also  the 
lateral  thrust  of  the  cam-shafts  in  one  direction,  but  this 
is  successfully  met  by  the  use  of  collars.  The  two-piece 
or  sectional  collar  will  be  found  superior  to  the  single- 
piece  collar,  as  it  has  a  clamping  effect  in  addition  to 
that  of  the  set-screws. 

Where  there  is  a  minimum  of  jar  in  the  battery-posts, 
and  the  bearing-boxes  are  not  liable  to  get  out  of  line, 
cast-iron  cam-shaft  boxes  are  the  best,  as  there  is  no 
loss  of  time  in  babbitting,  and  little  trouble  from  the 
shaft  getting  out  of  alignment  or  from  having  an  abnor- 
mal strain  due  to  the  wearing  and  breaking  away  of 
the  babbitt.  Where  there  is  much  vibration  of  the  bat- 
tery-posts, babbitt,  being  a  softer  metal,  is  preferred  for 
holding  the  shaft,  while  the  frequent  re-babbittings  serve 
for  aligning  the  shaft  anew. 

To  babbitt  the  boxes,  the  shaft  is  raised  by  means  of 
screw-jacks,  chain-blocks,  or  preferably  by  two  long 
pieces  of  timber  used  as  levers.  The  old  babbitt  is 
knocked  out  by  means  of  chisels,  when  the  shaft  is  let 
down  into  the  position  in  which  it  shall  run.  Cardboard 


48  STAMP  MILLING  AND  AMALGAMATION 

luted  with  clay  is  placed  about  the  shaft  at  the  ends  of 
the  boxes,  so  that  the  molten  metal  may  not  run  out. 
The  melted  babbitt  is  now  poured  in,  and  as  soon  as 
cold  the  clay  is  removed,  and  also  the  timber  or  other 
supports  to  the  shaft,  after  which  the  shaft  is  started 
revolving  and  the  stamps  to  dropping.  Should  the  bab- 
bitt give  trouble  by  breaking,  especially  in  the  box  at  the 
opposite  end  of  shaft  from  the  pulley,  it  should  be  soft- 
ened by  adding  some  lead  in  the  melting.  It  is  well  to 
have  on  hand  a  set  of  half-rings  made  of  iron.  These 
are  inserted  at  each  end  of  the  boxes,  and  the  shaft  is 
lowered  on  them,  and  should  be  kept  perfectly  level, 
after  which  the  supports  are  removed.  The  rings  serve 
to  hold  the  shaft  in  position  while  the  babbitt  is  being 
poured  in  and  to  make  the  shell  of  babbitt  of  the  right 
thickness.  It  may  also  be  necessary  to  use  the  clay  to 
keep  the  molten  babbitt  from  running  out.  After  the  bab- 
bitt has  set,  the  rings  are  pried  out.  Iron  caps  for  these 
bearings  are  universally  discarded,  but  dust-caps  of  can- 
vas should  always  be  used. 

When  a  10-cam  shaft  breaks,  running  is  continued  with 
5  stamps  if  possible,  until  ready  to  remove  the  shaft. 
The  stamps  are  then  raised  by  the  chain-blocks  until  a 
6-in.  block  can  be  slipped  between  each  tappet  and  its 
finger-jack.  This  permits  the  shaft  to  be  raised  by  levers, 
screw-jacks,  or  chain-blocks,  first  with  chain-blocks  re- 
moving the  pulley,  to  be  rolled  out  on  timbers  away  from 
the  boxes.  If  self-tightening  cams  are  used,  they  are  re- 
moved, as  it  is  only  necessary  to  strike  them  with  a 
hammer  on  the  point  in  the  reverse  way  to  which  they 
run.  The  broken  shaft  is  taken  away  and  a  new  one  is 
brought.  The  cams  are  placed  on  this  shaft,  and  it  is 


ORDER  OF  DROP  49 

then  rolled  and  lowered  into  place.  If  the  old-fashioned 
cams,  fastening  with  keys  in  a  slot  cut  in  the  shaft,  are 
used,  the  shaft  and  cams  are  at  once  removed  from  the 
mill  and  a  new  shaft,  having  a  full  set  of  cams  in  position, 
is  placed  in  the  bearings.  This  method  is  necessitated  by 
the  fact  that  to  remove  and  to  replace  a  set  of  keyed 
cams  is  a  long  and  laborious  process.  All  well-managed 
mills  keep  on  hand  at  least  one  extra  cam-shaft  with 
cams,  and  several  extra  stems. 

The  order  of  drop  of  the  stamps  in  a  battery  should  be 
such  that  an  even  bed  of  pulp  is  kept  over  the  dies,  rather 
than  an  excess  over  one  die  and  too  little  over  another, 
and  that  the  splashing  or  wave  motion  of  the  pulp  be 
produced  evenly  along  the  screen.  The  first  has  reference 
to  the  crushing  efficiency  of  a  battery,  while  the  second 
refers  to  its  screening  capacity,  the  two  factors  that  make 
for  tonnage.  The  academic  requirement  that  no  two  ad- 
jacent stamps  shall  drop  consecutively,  or  in  simpler  lan- 
guage, that  no  two  stamps  side  by  side  shall  follow  each 
other  in  dropping,  is  fulfilled  by  only  one  order,  1-3-5-2-4, 
or  its  reverse  which  is  usually  spoken  of  as  1-4-2-5-3, 
since  the  custom  in  numbering  is  to  face  the  front  of  the 
mortar  and  to  consider  the  first  or  end-stamp  on  the  left 
as  dropping  first.  This  is  the  drop  usually  recommended. 
Another  order  of  drop  and  one  which  is  most  popular  with 
practical  millmen  is  1-5-2-4-3.  It  will  be  noticed  that  the 
third  stamp  follows  the  fourth  in  dropping.  The  reverse 
of  this  drop  as  determined  by  counting  it  from  the  rear 
of  the  mortar  and  then  applying  it  to  the  front  is  1-4-2-3-5, 
in  which  the  third  stamp  follows  second  in  dropping. 

Practically  every  conceivable  order  of  drop  has  been 
used,  but  the  above  two  systems  are  the  only  ones  that 


50  STAMP  MILLING  AND  AMALGAMATION 

have  stood  the  test  of  time.  Much  confusion  exists  in 
speaking  or  writing  of  the  different  orders  of  drop.  Thus 
one  man  will  say  that  1-3-5-2-4  is  a  good  order  and  has 
been  found  to  be  satisfactory,  and  that  1-4-2-5-3  has  been 
found  to  be  a  poor  order  and  unsatisfactory.  Since  the 
latter  is  the  reverse  of  the  first,  it  is  difficult  to  under- 
stand how  it  could  be  better  or  worse. 

The  1-5-2-4-3  order  has  been  found  superior  to  the 
1-3-5-2-4,  both  generally  and  where  they  have  been  tested 
against  each  other.  A  case  under  observation  will  illus- 
trate the  difficulties  and  the  disadvantages  of  the  1-3-5-2-4 
order.  There  was  in  use  a  narrow  mortar  and  a  1000-lb. 
stamp  dropping  103  times  per  minute  through  a  distance 
of  7  to  8  in.  As  long  as  the  height  of  discharge  was  kept 
as  low  as  possible,  and  the  feed  preferably  of  coarse  rock, 
little  trouble  was  experienced.  The  tendency  of  the  pulp 
to  bank  under  the  first  stamp  and  leave  the  fifth  pounding 
was  marked,  but  in  proportion  as  the  feed  was  kept  low 
and  the  stamps  almost  *  pounding  steel,'  this  trouble  was 
overcome.  As  the  height  of  discharge  was  raised  and  finer 
rock  was  fed  to  the  mortar,  the  trouble  from  the  pulp 
swinging  toward  the  first  stamp  became  so  great  that  when 
attempting  to  do  fine  crushing  by  the  use  of  a  high  dis- 
charge and  a  fine  screen,  the  results  were  most  unsatis- 
factory, both  as  to  tonnage  and  operation.  The  first  stamp 
was  set  to  drop  8  in.  and  the  others  evenly  graded  down 
to  4%  in.  on  the  fifth  stamp,  but  without  getting  the 
stamps  to  strike  a  blow  of  equal  hardness.  The  pulp 
discharged  from  the  third,  fourth,  and  mainly  from  the 
fifth  stamp,  so  that  it  was  necessary  to  improvise  a  dis- 
tributing box  to  get  an  even  distribution  across  the  plates. 
This  difficulty  with  the  1-3-5-2-4  order,  when  running 


ORDER  OF  DROP  51 

with  a  medium  or  high  discharge,  is  generally  reported. 

The  1-5-2-4-3  order  gives  a  more  even  splash  across  the 
screen  and  a  better  distribution  in  the  mortar;  conse- 
quently it  gives  a  higher  capacity  with  less  trouble  in 
operating.  It  scours  more  severely  in  the  centre  of  the 
mortar  than  the  other  order,  so  that  it  is  harder  on  the 
screen  and  chuck-block  at  this  point. 

While  recommending  that  the  1-5-2-4-3  order  be  used, 
it  is  advisable  that  the  millman  be  able  easily  to  change 
to  the  1-3-5  2-4  and  thus  try  both.  The  mills  built  today 
are  all  supplied  with  self-tightening  cams,  as  these  have 
been  so  satisfactory  that  no  one  would  think  of  going  back 
to  the  old-fashioned  keyed  cam;  these  automatically  lock 
themselves  into  position  according  to  a  clip  on  the  cam- 
shaft. The  position  of  this  clip  is  determined  by  two  holes 
bored  in  the  shaft  in  which  the  lugs  of  the  clip  are  set. 
By  drilling  two  extra  sets  of  holes  in  the  cam-shaft,  mak- 
ing seven  sets  to  a  battery  instead  of  the  usual  five,  it 
will  be  possible  to  change  from  one  order  of  drop  to  the 
other.  Such  a  boring  would  give  the  drops  1-3-5-2-4  and 
3-1-5-2-4.  It  will  be  observed  that  the  3-1-5-2-4  order  is 
the  1-5-2-4-3  with  the  numbering  commencing  at  the 
third  stamp  to  enable  a  simpler  comparison  to  be  made 
with  the  other  order.  To  change  from  one  order  to  the 
other,  it  would  only  be  necessary  to  reset  the  first  two 
cams  to  the  other  positions;  a  thing  that  can  be  done 
easily  and  quickly.  The  millman  now  has  actually  the 
first,  and  in  theory,  the  second  of  the  two  orders  of  drop 
comprised  in  each  of  the  two  systems  spoken  of,  namely, 
the  first  system:  1-3-5-2-4,  and  1-4-2-5-3;  and  the  second 
system :  1-5-2-4-3,  and  1-4-2-3-5. 

Guides  should  be  of  cast  or  malleable  iron  and  of  the  in- 


52  STAMP  MILLING  AND  AMALGAMATION 

dividual  type,  though  wooden  guides  are  much  used  in 
the  older  mills.  They  should  be  bored  to  a  close  fit  of 
Vs2  to  1/64  in.  larger  in  diameter  than  the  stem,  carefully 
aligned  and  adjusted  when  first  run,  and  supplied  with 
a  good  grade  of  lubricating  oil  instead  of  the  usual  dirty 
oil  or  scrap-grease.  They  should  be  kept  as  tight  as  pos- 
sible without  heating  or  rubbing  so  that  the  stamp  may 
move  truly  up  and  down  with  little  side-play.  The 
wooden  guide  can  never  give  a  close  fit  without  heating, 
and  sooner  or  later  there  will  be  considerable  side-play 
from  the  wearing  away  of  the  wood. 

The  finger- jacks  should  raise  the  tappets  %  in.  clear 
of  the  cams,  so  that  a  cam-stick  of  three  and  not  to  ex- 
ceed four  thicknesses  of  heavy  belting  may  be  used. 
Thicker  cam-sticks  are  too  heavy  to  handle  with  ease, 
and  are  more  liable  to  be  cut  to  pieces  by  a  stamp  with  too 
long  a  drop.  One  very  thick  cam-stick  should  be  kept 
on  hand  to  increase  the  height  of  drop  of  the  stamps  in 
putting  in  shoes  or  stems  and  in  pounding  out  choked 
mortars.  A  box  3  by  6  in.  and  9  in.  deep  should  be  fitted 
to  the  central  battery-post  of  each  cam-shaft  for  holding 
the  cam-stick.  Cam-sticks  made  of  iron  with  handles  of 
leather  or  belting,  work  well,  but  care  must  be  used  in 
placing  them  on  the  cams  or  they  will  fly  back  in  a  dan- 
gerous manner.  The  finger-jacks  should  be  solid  and 
steady;  a  stamp  resting  on  a  wobbly  finger-jack  a  frac- 
tion of  an  inch  too  short,  and  in  connection  with  loose 
guides,  is  a  dangerous  thing  beneath  which  to  examine 
the  interior  of  a  mortar. 

The  suspended  type  of  the  Challenge  feeder,  or  one 
of  the  so-called  ' improved'  feeders  of  the  same  order, 
is  now  almost  universally  used,  as  it  gives  a  free  floor 


AUTOMATIC  FEEDERS  53 

and  is  less  in  the  way  than  the  platform  type.  However, 
the  standard  platform  type  of  Challenge  feeder  is  still 
the  strongest  and  most  satisfactory  working  machine 
made.  The  revolving  feed-plate  should  be  set  4  in.  above 
the  mouth  of  the  mortar,  so  that  a  platform  of  wood  or 
sheet-metal  may  be  attached  to  the  mortar  to  catch  as 
much  of  the  drippings  from  the  feeder  as  possible ;  and  also 
to  allow  of  the  introduction  of  a  long  tin  scoop  for  catch- 
ing a  sample  of  the  mill-feed  as  it  drops  off  the  revolving 
plate.  This  revolving  feed-plate  should  be  provided  with 
a  false  plate  or  liner  that  may  be  readily  replaced  when 
worn  out.  Feeders  were  formerly  actuated  by  a  bumper- 
rod  struck  by  a  cam  tappet,  but  as  these  rods  are  liable 
to  give  trouble  at  times,  they  are  not  included  in  the 
design  of  a  modern  mill;  a  small  'feed  tappet  *  on  the 
middle  stamp  striking  the  arm  of  the  feeder  is  now  used. 
This  tappet  should  be  split  longitudinally,  since  in  chang- 
ing a  feed  stem,  putting  on  the  feed-tappet  is  often  for- 
gotten until  after  the  stem  is  stuck  in  the  boss.  These 
tappets,  however,  frequently  give  trouble  by  slipping, 
due  to  grease  running  down  from  above,  or  to  the  bear- 
ing parts  being  too  smooth.  Feeders  are  the  only  parts 
in  a  stamp-mill  equipment  that  are  of  delicate  construc- 
tion, and  they  should  receive  careful  attention.  If  worn 
out  or  broken,  they  should  be  renewed  or  rebuilt,  as  poor 
feeders  cause  the  mill  employees  great  annoyance,  reduc- 
ing the  mill  capacity  and  increasing  the  breakages. 

Screens  are  made  of  perforated  metal  plate  or  of  woven 
wire  or  so-called  *  cloth.'  Perforated  screens  are  either 
plain  or  burr-punched;  in  the  first  case  a  piece  of  the 
metal  is  punched  out  to  make  the  hole,  while  in  the  sec- 
ond the  metal  is  bent  inward  instead  of  being  removed. 


54  STAMP  MILLING  AND  AMALGAMATION 

Burr-punched  screens  require  to  be  placed  with  the  burr 
or  ragged  edge  inside  the  mortar  so  that  the  grains  of 
pulp  may  not  wedge  in  the  orifices,  and  thus  reduce  its 
capacity.  The.  sheet-steel,  whether  punched  with  round 
or  slot-openings,  is  the  strongest;  its  life  is  so  long  that 
it  must  sometimes  be  removed  before  breaking  on  account 
of  the  openings  becoming  enlarged  by  wear.  The  slot- 
screen  gives  less  trouble  from  clogging  than  the  round 
punched.  Even  the  perforated  screens  have  a  slight  burr 
on  one  side,  which  should  be  turned  inside  the  mortar. 
The  Russia  iron  needle-punched  screen  is  used  to  quite  an 
extent,  though  it  does  not  facilitate  discharge  as  does  the 
wire  screen.  Brass-wire  screens  have  been  found  satis- 
factory for  fine  crushing.  They  cannot  be  used  when 
crushing  in  cyanide  solution,  as  the  screen  is  attacked  by 
the  solution,  and  soon  breaks.  Contrary  to  the  usual 
opinion,  they  do  not  amalgamate  to  a  prohibitive  extent. 
After  receiving  the  treatment  usually  accorded  old 
screens  for  removing  any  adhering  amalgam,  they  can  be 
melted  into  a  bar.  Where  a  stronger  screen  is  required, 
as  when  crushing  with  a  low  discharge,  iron  and  steel- 
wire  screens  are  used. 

A  most  satisfactory  screen  is  made  of  'tinned  iron.' 
This  coating  prevents  its  rusting  before  being  put  into 
use,  and  may  prevent  an  acid  battery-water  or  pulp  from 
attacking  the  screen  later.  It  has  also  been  suggested 
that  the  coating  of  soft  tin  protects  the  screen  from  the 
impact  and  attrition  of  the  pulp  by  presenting  a  yielding 
malleable  surface.  Some  millmen  remove  the  coating  by 
heating  the  screen  to  redness  over  a  forge  or  gasoline 
burner;  which  is  supposed  to  strengthen  the  screen 
by  annealing  it.  As  it  oxidizes  the  surface  of  the  iron, 


BATTERY  SCREENS  55 

it  should  not  be  done  where  the  battery-water  or  pulp  is 
acid.  Experiments  can  easily  be  conducted  on  the  same 
screen-frame  to  prove  the  value  of  annealing.  The  tinned- 
iron  screen  gives  the  least  trouble  from  clogging.  This 
is  because  of  the  thinness  of  the  metal.  A  grain  of  quartz 
that  will  lodge  in  a  hole  in  a  Russia  iron  or  steel-plate 
screen,  will  pass  the  same  size  of  opening  in  a  tinned-iron 
screen,  or  will  be  jarred  through  by  the  splashing  pulp 
within.  The  tinned-iron  has  been  given  preference  in 
many  mills  after  being  tested  against  the  thicker  perfor- 
ated and  the  woven-wire  screens.  The  character  of  the 
rock  is  one  of  the  determining  factors  in  the  choice  of 
screen  to  be  used.  The  tendency  to  clog  with  a  hard, 
splintery  quartz  is  great,  and  it  is  sometimes  possible  to 
run  a  40-mesh  tinned-iron  screen  on  an  ore  where  it  is 
impossible  to  use  one  of  woven-wire  that  is  finer  than  30- 
mesh  on  account  of  the  clogging. 

When  crushing  through  to  40-mesh,  the  brass  wire  is 
a  good  screen  to  use.  Should  it  clog  much,  the  tinned- 
iron  screen  should  be  tried.  For  crushing  between  16  and 
30-mesh,  the  tinned-iron  is  satisfactory,  except  where  the 
discharge  is  carried  too  low  to  allow  these  screens  to  have 
a  reasonable  length  of  life,  when  the  diagonal-slot  thin 
steel  screen  may  be  substituted.  For  crushing  to  12  or 
16-mesh  with  a  low  discharge,  the  diagonal-slot  thin  steel 
screen  will  probably  give  the  best  results.  The  value  of 
woven-wire  screens  increases  as  their  tendency  to  clog 
becomes  less.  Steel  screens  of  special  thinness,  and  the 
needle-punched  type,  have  been  tried  against  the  tinned- 
iron  with  satisfactory  results,  but  have  not  come  into 
general  favor. 

When   screens   clog,   they   are   scraped,   brushed,    and 


56  STAMP  MILLING  AND  AMALGAMATION 

slapped  in  an  effort  to  keep  them  clear.  Wire  screens 
are  removed  when  badly  clogged  and  left  to  become  dry, 
when  they  are  brushed  and  slapped.  Tinned-iron  screens 
are  not  as  strong  as  the  iron  or  steel-wire  or  plate-screens, 
but  they  are  cheaper.  Perforated  screens  usually  break 
close  to  the  screen-frame.  In  such  a  case  a  piece  of  wood 
1%  in.  square,  and  slightly  longer  than  the  break,  is  cov- 
ered with  canvas  or  a  piece  of  blanket  on  two  sides,  and 
attached  to  the  frame  by  one  or  two  short  nails  to  cover 
the  break  until  it  is  convenient  to  remove  or  turn  the 
screen-frame. 

A  strip  of  canvas  or  gasket-rubber  between  the  screen 
and  the  frame  will  prolong  the  life  of  the  screen.  A  good 
way  in  which  to  attach  screens  to  the  frame  is  to  punch 
them  over  a  template  to  fit  small  bolts  in  the  screen- 
frame,  bolting  in  place  by  means  of  strap-iron,  using  a 
hand  socket  wrench  for  the  nuts.  Strap-iron  at  the  top 
and  bottom  of  a  screen  is  a  good  protector.  Some  success 
has  been  attained  by  having  two  heights  of  screen-frames, 
so  that  when  the  screen  is  well  worn  at  the  top  and  bot- 
tom, after  turning  the  screen-frame,  it  may  be  transferred 
to  a  narrower  frame,  the  worn  parts  now  coming  in  con- 
tact with  the  frame. 

It  has  been  found  advisable  to  set  the  screen  in  the 
mortar  at  an  angle  from  the  vertical,  in  order  that  the 
pulp,  besides  being  driven  through  the  screen,  may  fall 
on  it  and  run  through  as  it  flows  down  after  the  splash. 
An  angle  of  10  to  13°  from  the  vertical  has  been  consid- 
ered sufficient.  It  would  appear  that  the  proper  angle  of 
inclination  of  the  screen  would  be  dependent  upon  the 
manner  of  operating  the  battery,  whether  using  a  high 
or  a  low  discharge. 


CHAPTER  IV. 

The  feed-water  for  a  battery  should  be  introduced  into 
the  top  or  into  the  feed-mouth  of  the  mortar,  using  a  pipe 
on  each  side  because  it  may  be  thought  necessary  to  intro- 
duce more  water  toward  one  end  of  the  battery  than  the 
other.  The  valves  should  be  accessible  from  the  front  of 
the  mortar,  and  there  should  be  two  to  each  pipe;  one 
being  a  globe-valve  by  which  the  exact  quantity  of  water 
used  is  regulated,  and  the  other  bibb  or  plug  cock.  When 
it  is  necessary  to  shut  off  the  feed-water,  the  plug  cock 
is  used,  and  when  starting  again,  it  is  thrown  wide  open. 
No  time  is  thus  lost  in  adjusting  the  amount  of  water, 
as  that  is  provided  for  by  the  globe-valve.  The  water- 
supply  should  come  from  a  tank  having  a  constant  head, 
for  there  should  be  no  variation  in  amount  of  water 
flowing  over  the  plates.  Where  the  water  is  returned  for 
re-use,  or  is  received  in  a  storage  tank,  this  tank  should 
have  large  area  in  order  to  avoid  a  rapid  reduction  in  the 
head.  The  main  water-supply  pipes  entering  the  mill  and 
running  the  length  of  the  batteries  should  be  of  large  dia- 
meter so  that  the  amount  of  water  passing  into  one  mortar 
may  not  be  decreased  or  increased  by  starting  or  stopping 
the  flow  into  the  other  mortars.  When  crushing  in  cya- 
nide solution,  the  pipes  should  be  of  large  diameter  and 
easily  taken  apart  as  they  become  gradually  encrusted. 
Launders  have  been  used  instead  of  pipes. 

Attempts  have  been  made  to  introduce  the  water  in  the 
front  or  rear  of  the  mortars  and  on  a  level  with  or  just 
below  the  tops  of  the  dies.  The  arguments  advanced  are 


58  STAMP  MILLING  AND  AMALGAMATION 

that  the  finer  material  is  thus  floated  up  and  out  pf  the 
mortar,  and  that  the  pulp  just  below  the  face  of  the  die 
is  kept  active,  permitting  the  amalgam  to  sink  into  it  and 
be  caught.  It  would  require  a  higher  head  of  water  than 
can  ordinarily  be  obtained,  and  in  fact  higher  than  it  is 
desirable  to  use,  to  overcome  the  violent  pulsations  im- 
parted to  the  pulp  by  the  falling  stamps  and  to  give  a 
classifying  effect  in  the  mortar;  and  should  it  overcome 
these  pulsations,  the  result  would  be  to  interfere  with  the 
even  distribution  of  the  pulp  over  the  dies.  On  account 
of  the  wearing  of  the  dies,  it  is  impossible  that  the  feed- 
water  should  enter  at  the  proper  point  in  relation  to  the 
face  of  the  dies  for  any  great  length  of  time.  Where  the 
water  has  been  introduced  in  the  rear  of  the  mortar,  it 
has  been  found  that  when  the  dies  are  nearly  worn  out 
and  the  screen  is  consequently  set  low,  the  water  shoots 
across  the  mortar  and  through  the  screen.  Furthermore, 
it  is  almost  impossible  to  maintain  a  water-tight  connec- 
tion between  a  pipe  and  a  mortar. 

Should  the  feed-water  be  shut  off,  the  stamps  must  be 
hung  up  with  all  possible  speed,  for  they  will  sink  down 
through  the  pulp  to  the  die  and  continue  falling  until 
the  mortar  is  full.  Owing  to  the  absence  of  water,  the 
pulp  does  not  run  out  nor  splash  back  under  each  stamp. 

The  amount  of  feed-water  used  in  a  mortar  is  gauged 
by  the  flow  of  the  pulp  over  the  plates,  the  water  being 
used  in  such  quantities  as  to  give  ideal  conditions  for 
amalgamating  on  the  plate-tables,  rather  than  to  supply 
the  quantity  that  will  give  the  greatest  crushing  and 
screening  effect  in  the  mortar.  The  amount  of  water  used 
per  ton  of  ore  stamped  varies  from  4  to  10  tons.  Where 
effective  amalgamation  takes  place  on  a  short  apron-plate, 


HEIGHT  OF  DROP  59 

6l/2  tons  will  be  about  the  average  amount.  The  crushing 
capacity  of  a  battery  increases  with  the  amount  of  water 
used,  up  to  the  point  where  a  good  splash  or  wave  motion 
on  the  screen  can  no  longer  be  secured.  This  increase  in 
capacity  is  more  noticeable  in  a  mortar  with  a  deep  dis- 
charge than  with  a  shallow  one,  for  such  a  mortar  sizes 
and  discharges  to  a  much  greater  extent  than  one  with  a 
low  discharge.  If  large  quantities  of  water  are  used  the 
amalgamation  is  usually  not  so  effective,  long  outside 
plates  and  auxiliary  amalgamating  devices  then  being 
required. 

The  height  of  drop  to  be  given  the  stamps  depends  on 
the  size  and  hardness  of  the  ore,  on  the  weight  of  the 
stamps,  and  to  some  extent  on  the  treatment  required  for 
the  ore.  Hard  ore  will  require  a  heavier  blow  and  con- 
sequently a  longer  drop  than  soft  ore.  Similarly  a  small 
piece  of  ore  will  not  require  as  hard  a  blow  or  as  long  a 
drop  as  when  coarse.  Consequently  a  hard,  tough  ore 
should  be  broken  finer  in  the  breaker  than  soft,  brittle, 
friable  material.  The  stamp-battery  does  not  crush  alto- 
gether by  catching  each  particle  of  rock  between  the  shoe 
and  die,  but  mainly  by  the  attrition  of  the  particles  of 
rock  upon  each  other  flying  from  the  impact  of  the  falling 
stamps.  This  is  the  reason  why  the  limit  of  economy  can 
be  passed  in  fine  crushing  in  the  rock-breaker.  It  would 
appear  that  the  size  to  which  the  ore  should  be  broken  in 
the  breaker  would  have  some  relation  to  the  thickness  of 
the  bed  of  pulp,  but  in  actual  practice  the  softness 'and 
nature  of  the  ore  is  the  determining  factor.  Millmen  pre- 
fer to  have  the  ore  broken,  to  what  may  be  termed  'a 
medium  coarse  size,'  rather  than  pulverized  fine,  as  such 
ore  feeds  better  and  causes  the  battery  to  work  more 


60  STAMP  MILLING  AND  AMALGAMATION 

evenly.  Also  some  ores  that  are  soft  or  brittle  rather 
than  hard,  tough,  and  close  grained,  appear  to'  crush 
faster  when  containing  coarse  material  which  increases 
the  attrition.  This  point  should  be  investigated  in  de- 
termining the  proper  size  for  preliminary  crushing  in  the 
breaker.  To  crush  a  hard  rock  to  a  size  approximating 
%  in.  diam.,  and  a  soft  rock  to  1%  in.  would  appear  ideal, 
but  in  practice  it  is  all  crushed  to  a  maximum  diameter 
of  ll/2  to  2%  inches. 

For  a  hard  ore  a  drop  of  7  to  10  in.  is  usual;  for  a 
medium  ore  from  6  to  8  in.,  and  for  a  soft  ore  from  4% 
to  6%  in.  Increasing  the  weight  of  the  stamps,  and  break- 
ing the  rock  finer  in  the  breaker  permits  a  shorter  drop 
to  be  used.  As  the  height  of  drop  is  lessened,  the  stamps 
should  be  run  faster  on  the  principle  that  they  should 
drop  as  fast  as  possible;  the  increased  speed  thus  offsets 
the  loss  of  crushing  power  through  shortening  the  length 
of  drop.  A  short  drop  and  higher  speed  indirectly  in- 
creases the  capacity  by  keeping  the  finer  material  in  bet- 
ter suspension  and  by  washing  it  out  of  the  mortar  faster. 
Too  short  a  drop  may  not  allow  the  die  to  become  covered 
with  pulp.  The  speed  at  which  stamps  can  be  run  safely 
has  been  worked  out  mathematically,  but  the  millman 
desirous  of  maintaining  a  high  tonnage  will  run  the  stamps 
as  fast  as  possible  up  to  the  point  where  the  tappet  just 
stops  short  of  falling  on  the  cam  when  the  stamp  is  at  its 
maximum  height  of  drop. 

In  the  older  mills  the  stamps  weighed  500  to  750  Ib. ; 
this  has  been  increased  until  in  America  the  weight  of  a 
standard  stamp  with  a  new  shoe  is  1000  Ib.  Extended  ex- 
periments made  in  some  large  mills  have  indicated  that 
to  be  the  best  weight.  The  reason  given  is  that  a  heavier 


WEIGHT  OF  STAMPS  61 

stamp  crushes  more  ore  than  the  plates  can  handle.  In 
South  Africa  the  tonnage  has  increased  with  the  weight 
of  the  stamps,  the  favorite  weight  of  stamps  now  being 
1500  to  1750  Ib.  Capacities  higher  than  10  tons  per  stamp 
with  coarse  crushing  are  reported.  A  large  mill  is  now 
under  construction  that  will  have  2000-lb.  stamps.  At- 
tempts have  been  made  to  show  theoretically  that  this  is 
beyond  the  economic  weight  for  a  stamp,  but  only  actual 
experience  can  demonstrate  this. 

The  weights  of  the  different  parts  of  a  stamp  are  ap- 
proximately :  stem,  43% ;  tappet,  15 ;  boss,  26 ;  shoe,  16. 
The  weight  of  a  stamp  may  be  increased  in  two  ways,  by 
placing  an  extra  tappet  on  the  stem,  either  above  or  below 
the  upper  guide,  or  by  using  a  false-shoe;  this  shoe  is 
identical  with  the  regular  shoe  used,  with  the  exception 
that  it  has  a  socket  similar  to  the  shoe-socket  of  the  boss. 
The  false-shoe  is  first  put  on  in  the  usual  way,  after  which 
the  crushing  shoe  is  put  on.  It  has  been  impossible  to  get 
millmen  to  take  an  interest  in  any  of  these  ways  of  in- 
creasing the  weight  of  a  stamp  when  the  shoe  is  worn 
down,  though  the  decrease  in  capacity  is  readily  notice- 
able, especially  with  the  lighter  stamps. 

What  determines  the  weight  of  stamp  to  be  used  ?  Prop- 
erly, the  hardness  of  the  ore  and  the  tonnage  desired,  but 
ordinarily,  custom.  That  custom  regulates  the  weight  is 
proved  by  the  fact  that  though  it  has  been  fully  demon- 
strated that  the  tonnage  increases  with  the  weight  of  the 
stamps,  hardly  any  attempt  has  been  made  to  increase 
their  weight  in  the  many  installations  made  in  the  past 
few  years  where  amalgamation  is  not  practised.  Most  of 
these  stamps  have  weighed  1000  Ib.,  a  few  1250,  and  more 
rarely  1500.  Even  if  amalgamation  is  to  be  practised, 


62  STAMP  MILLING  AND  AMALGAMATION 

heavy  stamps  should  be  employed,  as  some  system  of 
handling  the  pulp  can  always  be  devised.  Heavy  stamps 
cost  so  little  more  to  install  and  operate,  that  their  in- 
creased tonnage  capacity  comes  almost  as  a  gift.  A  stamp 
lighter  than  1000  Ib.  should  only  be  ordered  for  an  ex- 
tremely soft  ore,  for  should  the  stamp  break  through  the 
bed  of  pulp  and  strike  the  die,  a  shorter  drop  can  be  used. 
The  heavy  stamp  is  well  adapted  for  coarse  crushing,  and 
with  the  increasing  use  of  Chilean  and  tube-mills  for  fine 
grinding  it  may  be  expected  that  heavier  stamps  will  be 
used.  There  is  no  reason  why  a  modern  stamp-mill  should 
not  be  equipped  with  1500-lb.  stamps,  and  arrangements 
made  to  divide  the  pulp  between  two  amalgamating  tables 
placed  one  in  front  of  the  other. 

The  capacity  of  a  first-class  modern  stamp-battery  in 
America  can  be  estimated  at  4  tons  per  24  hours  through 
a  30-mesh  screen.  There  are  so  many  varying  factors  that 
the  tonnage  ranges  from  3  to  6  tons,  but  4  will  be  found 
the  average  capacity. 

The  height  of  discharge  to  be  used  depends  upon 
whether  coarse  or  fine  crushing  is  to  be  done ;  whether  an 
attempt  is  made  to  catch  much  of  the  gold  in  the  mortar 
or  not ;  whether  it  is  desired  to  prepare  the  gold  for  amal- 
gamation by  keeping  it  longer  in  the  mortar,  as  with 
'rusty'  gold  requiring  abrasion;  also  whether  capacity  is 
desired  or  the  crushing  is  being  done  for  concentration. 

When  the  discharge  is  kept  as  low  as  possible  without 
punching  or  wearing  out  the  screen  too  fast,  that  is,  with 
a  1%  to  2-in.  discharge,  the  greatest  capacity  results,  and 
the  sizing  is  more  evenly  done  up  to  the  point  where  the 
screen  openings  are  enlarged  by  the  violence  with  which 
the  pulp  is  forced  through  them.  The  minimum  of  sliming 


HEIGHT  OF  DISCHARGE  63 

is  then  done,  and  consequently  the  low  discharge  is  the 
best  for  concentrating  purposes,  as  it  affords  the  best  op- 
portunity for  the  sulphide  liberated  from  the  gangue  to 
be  discharged  from  the  mortar  as  soon  as  crushed  to  the 
screen  size,  instead  of  being  crushed  and  slimed  still  finer, 
as  would  be  the  case  with  a  high  discharge.  The  loss  in 
concentration  is  mainly  in  the  slimed  sulphide. 

As  the  height  of  discharge  is  raised,  the  pulp  becomes 
finer,  is  slimed  more  in  comparison  to  the  size  of  the  screen 
used,  and  is  more  unevenly  sized.  A  large  amount  of 
pulp  is  retained  in  the  mortar,  perhaps  two  or  three  times 
as  much  as  with  a  low  discharge,  so  that  the  gold  remains 
longer  subjected  to  the  action  of  the  stamps,  and  has  less 
prolonged  contact  with  the  quicksilver  and  inside  plates. 
The  gold  also  receives  more  abraiding  and  polishing  by 
the  stamps  and  pulp,  which  may  or  may  not  be  an  advan- 
tage, depending  upon  the  amenability  of  the  gold  for  amal- 
gamation. As  this  pulp  is  hurled  less  violently  through 
the  screens,  and  washes  over  a  lesser  area  of  the  screen- 
surface,  and  consists  of  the  upper,  more  dilute,  finer 
portion,  the  capacity  is  reduced.  The  sulphide  from  its 
higher  specific  gravity  tends  to  settle  upon  the  die  and 
is  crushed  finer  than  the  gangue  from  which  it  is  liber- 
ated, in  proportion  to  the  height  of  discharge  and  the 
slowness  of  the  drop.  This  is  seen  in  the  Gilpin  county, 
Colorado,  practice  where  a  16-in.  drop,  with  30  drops  per 
minute,  and  a  13-in.  discharge,  together  with  a  wide 
mortar,  are  used  so  that  the  sulphide  may  be  thoroughly 
slimed  and  may  thereby  liberate  its  mechanically-held 
gold  for  amalgamation.  Where  concentration  is  to  fol- 
low crushing  the  exact  reverse  of  this  practice  is  used. 

A  high  discharge,  5  in.  at  least,  is  necessary  when  using 


64  STAMP  MILLING  AND  AMALGAMATION 

a  chuck-block  plate,  that  scouring  of  the  plate  may  not 
take  place.  Where  considerable  wood  enters  the  mortar 
with  the  ore,  a  low  discharge  is  sometimes  used  that  the 
wood  may  be  caught  under  the  stamps  and  thoroughly 
reduced  to  pulp  for  passing  through  the  screen;  or  a 
high  discharge  is  used  where  the  shoes  are  not  lifted  out 
of  the  water  so  that  the  wood  may  collect  in  a  line  along 
the  screen  and  be  removed  by  the  hand  or  a  straining 
spoon  of  wire  introduced  through  a  curtain  or  swinging 
wooden  door  above  the  screen. 

A  mortar  having  a  low  discharge  where  the  faces  of  the 
shoes  are  raised  out  of  the  water  and  the  pulp  is  violently 
splashed  over  the  screen  surface  is  called  a  'splash'  mor- 
tar or  battery;  while  a  mortar  having  a  high  discharge 
where  the  shoes  are  not  lifted  out  of  the  water  and  the 
pulp  runs  along  the  screen  in  waves  is  said  to  be  a  'wave' 
mortar  or  battery. 

In  feeding  a  battery  the  feed  should  be  kept  'low',  as 
can  be  ascertained  by  feeling  the  stem  as  the  shoe  strikes. 
Should  the  stamp  strike  a  well-cushioned  blow,  the  feed 
has  been  too  'heavy',  and  there  is  too  thick  a  bed  of  pulp 
on  the  dies  to  get  the  maximum  crushing  effect.  Should 
it  strike  with  a  jar,  or  rebound,  the  feed  has  been  too 
low,  and  there  is  too  thin  a  bed  of  pulp  on  the  dies  to  get 
the  maximum  crushing  effect,  or  to  prevent  the  shoe  and 
die  from  chipping,  and  the  life  of  the  stem  will  also  be 
shortened.  The  stamp  should  strike  a  hard,  firm  blow, 
just  barely  cushioned  on  the  pulp,  without  jar  or  re- 
bound. The  beginner  in  feeding  should  first  set  the  feeder 
so  that  it  works  at  every  drop  of  the  feed-stamp  without 
over-feeding  the  mortar,  then  by  adjusting  first  one  way 
and  then  another,  in  connection  with  feeling  the  stem, 


POWER  IN  THE  MILL  65 

a  point  will  be  found  where  it  is  apparent  to  the  eye  that 
the  maximum  that  the  stamps  can  crush  is  being  deliv- 
ered into  the  mortar.  By  feeling  the  stamps  now,  it  will 
be  found  that  they  are  barely  cushioned  on  the  pulp  from 
jar  and  rebound. 

The  power  necessary  for  operating  a  stamp-battery  is 
made  up  of  three  factors.  The  first  is  the  nominal  horse- 
power required  to  raise  the  stamps  without  reference  to 
friction;  this  is  the  power  directly  expended  in  crushing. 
By  remembering  that  a  horse-power  is  the  expenditure  of 
33,000  foot-pounds  of  energy  per  minute,  the  horse-power 
can  be  computed  at  any  time  without  the  use  of  a  formula 
by  multiplying  the  weight  of  the  stamp  by  the  length 
of  drop  in  feet  and  this  by  the  number  of  drops  per  min- 
ute, which  will  give  the  number  of  foot-pounds  expended 
in  lifting  the  stamp  during  a  period  of  one  minute,  and 
dividing  by  33,000.  As  the  stamp  appears  to  rise  slightly 
higher  than  its  drop,  as  measured  and  computed  when  at 
rest,  it  is  best  to  use  the  maximum  height  of  drop  rather 
than  the  average  in  computing. 

The  second  factor  is  the  power-demand  due  to  fric- 
tion— that  of  the  stem  in  its  guides,  which  is  small  and 
is  mainly  due  to  the  side-thrust  of  the  cam  striking  the 
tappet  away  from  the  centre  of  the  stem;  the  friction  of 
the  cam  on  the  tappet,  and  to  that  of  the  cam-shaft  in  its 
bearings.  This  power  requirement  is  somewhat  variable, 
but  according  to  the  generally  accepted  formula  of  Henry 
Louis,  it  amounts,  in  a  10-stamp  battery,  to  20%  of  the 
nominal  horse-power  required  to  raise  the  stamps.  The 
sum  of  these  two  powers  is  the  amount  that  must  be  ap- 
plied to  the  pulley  of  the  cam-shaft.  Special  attention  is 
called  to  the  fact  that  in  the  literature  dealing  with  the 


66  STAMP  MILLING  AND  AMALGAMATION 

computation  of  power  required  by  stamps,  reference  is 
made  to  this  power  only,  which  is  usually  spoken' of  as 
the  theoretical  horse-power. 

The  third  factor  is  the  power  consumed  by  the  friction 
of  the  driving  belt  between  the  line-shaft  and  the  cam- 
shaft, the  belt-tightener,  the  line-shaft  itself,  and  by  the 
belts  and  intermediate  shafting  between  the  line-shaft  and 
the  source  of  power.  The  power  consumed  in  the  driv- 
ing engine  or  motor  in  overcoming  its  own  inertia  and 
friction,  and  in  the  conversion  of  one  form  of  energy  into 
another,  may  be  included  in  this,  though  properly  it  should 
form  a  fourth  part.  This  third  part  of  the  power-  is  ex- 
tremely variable,  and  may  range  from  10  to  40%  of  the 
entire  power  consumed.  Consequently  the  greatest  care 
should  be  exercised  in  designing  and  constructing  a  mill 
and  in  selecting  the  machinery,  that  the  amount  of  power 
required  may  be  at  a  minimum  instead  of  expending  a 
large  part  in  uselessly  racking,  wearing,  and  tearing  the 
machinery  and  building.  The  abnormal  loss  of  power  in 
this  way  as  observed  in  stamp-mills,  outside  of  that  lost 
in  the  engine  or  motor  (the  type  and  size  of  which  needs 
to  be  carefully  looked  into),  may  occur  from  a  multi- 
plicity of  belts  and  shafts,  shafts  located  too  close  to- 
gether, and  of  too  small  a  diameter,  narrow  belts  and  pul- 
leys of  small  diameter,  loose  and  slipping  belts,  belt-tight- 
eners excessively  tight,  insecure  foundations,  and  un- 
stable framework.  When  a  bearing-box  is  running  warm, 
power  is  being  unnecessarily  consumed.  A  change  of  lu- 
bricants, or  a  change  to  ring  oilers  or  to  constant-drip  oil- 
cups,  may  effect  a  saving.  Where  a  mill  is  built  of  green 
timber,  the  boxes  will  need  occasional  readjustment,  ow- 
ing to  the  warp  of  the  timber.  Dust  from  the  breaker 


INDIVIDUAL  STAMP  67 

and  from  dumping  ore  in  the  bins  settles  in  the  cam-shaft 
boxes,  and  on  the  faces  of  the  cams  which  increases  fric- 
tion and  the  power  consumed. 

The  examination  and  correction  of  power-losses  is  a 
painstaking  task  and  is  often  neglected,  especially  in  the 
smaller  mills,  not  being  apparent,  and  through  not  real- 
izing the  amount  that  a  small  saving  in  this  direction  will 
reach  in  the  course  of  a  year. 

It  can  be  understood  that  it  is  impossible  to  give  any 
accurate  coefficient  that  will  give  the  sum  of  these  three 
parts  of  the  power,  or  of  the  actual  horse-power  con- 
sumed by  a  battery  having  an  independent  source  of 
power.  It  may  be  approximated  by  saying  that  in  a  good 
installation  of  say  40  stamps,  this  actual  horse-power  con- 
sumed will  amount  to  1.35  of  the  nominal  horse-power  re- 
quired to  raise  the  stamps',  but  that  in  a  small  installa- 
tion, or  where  the  efficiency  of  the  engine  or  motor  is  low, 
it  may  amount  to  more  than  this  figure. 

Various  types  of  'individual'  stamps  have  been  brought 
to  the  attention  of  the  mining  fraternity,  but  they  have 
been  generally  more  or  less  unsatisfactory,  partly  through 
inherent  defects  in  the  idea,  and  partly  through  the  gen- 
eral mill  details  being  improperly  worked  out.  The  first 
disadvantage  of  this  type  is  that  they  are  built  in  units 
of  2  or  3  stamps  each,  each  unit  requiring  the  same 
space  as  one  standard  5-stamp  battery.  A  summing  of  the 
extra  parts  required  to  make  up  the  equivalent  of  a  stan- 
dard 5-stamp  battery  with  individual  units  will  show  that 
the  cost  of  the  finished  individual-stamp  mill  may  be 
nearly  double  that  of  the  standard  type  of  the  same  num- 
ber of  stamps. 

The  great  argument  has  been  the  increased  screen  sur- 


68  STAMP  MILLING  AND  AMALGAMATION 

face.  This  cannot  be  denied,  but  in  answer  to  the  ques- 
tion as  to  whether  it  is  desirable,  or  not,  attention  is 
called  to  the  large  number  of  double-discharge  mortars 
in  use  with  their  back  discharges  closed  up.  In  the  5- 
stamp  mortar  there  are  from  500  to  550  blows  struck  per 
minute,  divided  evenly  all  over  the  mortar ;  this  results  in 
the  pulp  being  splashed  over  the  screen  surface  all  the 
time,  the  whole  length  of  the  screen  being  in  continuous 
use.  Whereas,  in  the  individual  mortar  there  is  only  one- 
fifth  the  number  of  blows  and  consequently  the  screen 
surface  is  not  in  continuous  use.  The  action  of  the  pulp 
within  the  mortar  should  be  carefully  studied  in  compar- 
ing the  individual  stamp  with  the  standard. 

With  the  quadruple-discharge,  there  are  four  screens 
and  one  feeder  to  each  stamp,  or  20  screens  and  five  feed- 
ers to  be  given  attention  for  each  5  stamps,  in  compari- 
son to  the  one  large  screen  and  feeder  of  a  standard  5- 
stamp  battery.  It  is  impossible  to  keep  an  even  height  of 
discharge  with  the  quadruple-discharge  mortar,  and  this 
results  in  severe  wear  and  tear  on  the  screens  before  the 
dies  are  worn  down.  They  overfeed  easily,  particularly 
when  running  on  fine  ore  and  using  a  short  drop;  this 
may  be  due  to  the  large  screen  area,  as  the  same  trouble 
has  been  noticed  in  the  double-discharge  5-stamp  mortar, 
or  to  the  fact  that  there  are  no  neighboring  stamps  to 
throw  the  pulp  back  on  the  die.  It  is  almost  useless  to  feed 
quicksilver  into  the  mortar  and  it  is  seldom  attempted. 

The  reports  of  those  using  these  stamps  in  actual  prac- 
tice is  that  they  give  little  if  any  increased  tonnage  over 
the  ordinary  stamps.  Where  they  have  been  run  to.  an 
advantage  has  been  with  a  large  quantity  of  water  and 
with  heavy  stamps,  conditions  that  are  enabling  the 


CAPACITY  PER  STAMP  69 

South  African  millmen  to  obtain  a  capacity  of  from  7 
to  9  tons  per  stamp.  The  good  showings  in  capacity  and 
absence  of  slime  has  been  made  by  using  a  minus  height  of 
discharge  and  with  so  much  trouble  from  screen  break- 
ages that  no  experienced  millman  would  attempt  to  run 
a  standard  battery  in  that  way. 

Until  the  claim  of  increased  capacity  is  positively 
proved,  it  will  be  contended  that  the  cost  for  power  per 
ton  crushed  is  higher  rather  than  lower  than  with  the 
standard  battery,  for  the  reason  that  the  cam-shaft  of 
the  individual  battery  is  identical  with  that  of  the  stan- 
dard, with  the  exception  that  it  raises  4  or  6  stamps  in- 
stead of  10.  Consequently  the  power  per  stamp  required 
to  drive  the  shafting,  independent  of  raising  the  stamps, 
in  the  individual  mill  is  from  1%  to  2%  times  that  re- 
quired for  the  standard  battery.  It  is  only  fair  to  add 
that  a  material  increase  in  the  capacity  would  offset  this 
increase  in  power  required  per  stamp. 

The  cost  of  operating  the  individual  stamp  is  much 
higher  than  with  the  standard.  This  is  due  to  the  extra 
cost  and  loss  of  time  from  screen  troubles  and  to  an  ex- 
cessive amount  of  general  repair  work.  Also  from  the 
fact  that  they  are  harder  to  operate  than  the  standard. 
In  a  certain  large  individual-stamp  mill  not  practising 
amalgamation,  and  where  the  arrangement  and  construc- 
tion is  good  and  operations  are  well-systematized — one 
batteryman  with  a  helper  attended  54  stamps — equivalent 
to  one  man  running  27  stamps.  Under  identical  condi- 
tions with  the  standard  type,  one  batteryman  would  be 
running  80  or  100  stamps.  At  another,  an  amalgamating 
mill  of  the  individual  type  having  18  stamps,  one  man 
was  barely  able  to  handle  the  mill  on  the  evening  and  mid- 


70  STAMP  MILLING  AND  AMALGAMATION 

night  shifts,  and  when  wet  ore  began  interfering  with  the 
operation  of  the  small  feeders,  two  men  were  required 
for  these  shifts. 

While  the  individual  stamp  is  a  unit  that  can  be  re- 
paired without  stopping  the  adjoining  stamps,  the  loss  of 
running  time  in  such  a  mill  is  actually  greater  than  in 
the  standard  mill,  due  to  the  extra  repairs,  changing 
screens,  and  other  work.  There  is  an  extra  loss  of  time 
through  the  increased  number  of  plate  dressings  required 
in  outside  amalgamation,  the  number  of  dressings  being 
double  that  required  when  quicksilver  is  fed  to  the  mor- 
tars. The  advantage  of  the  individual  mortar  is  that  the 
feed  of  each  stamp  can  be  regulated  to  get  the  greatest 
crushing  effect,  whereas  with  the  standard  battery  and 
a  lazy  millman,  a  few  stamps  in  a  battery  may  be  drop- 
ping hard  and  crushing  fast  while  the  others  are  being 
cushioned  and  crushing  little. 

In  crushing  for  concentration,  an  ore,  the  sulphides 
of  which  exhibit  a  tendency  to  slime,  the  individual  stamp 
may  be  an  advantage;  but  in  most  cases,  and  especially 
where  amalgamation  is  to  be  practised,  it  cannot  be  rec- 
ommended. 


Part  II 

AMALGAMATION 
CHAPTER  V 

Mercury  is  classed  as  a  metal,  and  is  unique  in  that 
it  is  liquid  at  ordinary  temperatures.  It  is  commonly 
called  quicksilver  on  account  of  its  color  and  activity; 
in  mills  it  is  also  spoken  of  as  the  'silver'  or  the  'quick'. 
It  is  13.6  heavier  than  water.  It  freezes  at  —  40°C.  It 
vaporizes  little  or  not  at  all  at  ordinary  temperatures,  but 
the  tendency  to  vaporize  increases  with  increase  of  tem- 
perature until  at  212  °F.  there  is  danger  of  salivation 
in  approaching  it.  It  boils  at  680°  F.  It  is  insoluble  in 
water,  but  violent  agitation  causes  a  little  of  it  to  be  taken 
up  mechanically  in  a  fine  state  of  division  by  the  water. 
It  combines  chemically  with  certain  substances  to  form 
two  series  of  compounds,  mercurous  and  mercuric.  It 
combines  mechanically  with  other  metals  to  form  alloys 
called  amalgams.  Mercury  is  readily  dissolved  by  strong 
nitric  acid,  and  slowly  by  the  dilute  acid.  It  is  not  dis- 
solved by  hydrochloric  acid.  It  is  dissolved  by  hot  con- 
centrated sulphuric  acid,  but  not  by  the  cold  acid.  Metal- 
lic mercury  is  slowly  dissolved  by  weak  or  strong  solu- 
tions of  potassium  cyanide  or  of  sodium  cyanide;  the 
rate  of  dissolution  increasing  with  the  strength  of  the 
solution;  the  compounds  of  mercury  are  more  readily 
attacked  by  these  solutions. 

Mercury  alloys  directly  with  most  of  the  metals  to  form 


72  STAMP  MILLING  AND  AMALGAMATION 

amalgams.  When  the  proportion  of  the  mercury  is  small, 
these  amalgams  are  hard,  solid,  and  crystalline;  as  the 
proportion  of  the  mercury  increases  the  amalgam  becomes 
pasty,  and  finally  liquid.  Gold,  silver,  copper,  lead,  zinc, 
tin,  cadmium,  bismuth,  tellurium,  sodium,  and  potassium 
unite  directly  with  mercury ;  the  latter  two  requiring  heat 
to  combine  actively.  Iron,  especially  when  in  a  fine 
state  of  division,  can  be  caused  to  unite  with  mercury 
by  means  of  sodium  amalgam.  Antimony  and  arsenic 
unite  with  mercury  when  heated.  Chromium,  mangan- 
ese, platinum,  aluminum,  and  nickel  will  unite  with  mer- 
cury by  the  employment  of  the  electric  current  and  by 
other  indirect  means. 

Pure  mercury  is  not  affected  by  the  air  at  ordinary 
temperatures,  but  when  impure,  its  surface  becomes 
coated  and  tarnished  with  compounds  of  the  base  metals, 
oxides,  sulphates,  chlorides,  and  sulphides,  and  probably 
to  some  extent  by  mercuric  oxide  and  sulphide.  Impure 
mercury  can  readily  be  recognized,  since  its  surface  will 
not  appear  bright,  nor  its  globules  spherical  and  tending 
to  quickly  unite  with  one  another  when  brought  together. 
When  rolled  about,  the  globules  will  be  sluggish  and  will 
elongate  to  a  tail  and  leave  a  black  film  behind.  Oils 
and  organic  substances  also  tend  to  render  mercury  im- 
pure. When  it  breaks  up  into  extremely  minute  globules, 
it  is  said  to  have  'floured',  and  will  readily  float  on  water 
and  when  these  refuse  to  coalesce  again,  the  mercury  is 
said  to  be  'sickened'.  Shaking  the  mercury  in  the  pres- 
ence of  detrimental  substances,  or  stamping  in  the  mortar 
energetically,  promotes  'flouring',  and  in  the  first  case 
'sickenening'  as  well.  Each  one  of  the  'sickened'  glob- 
ules of  mercury  is  surrounded  by  a  film  of  foreign  sub- 


TREATMENT  OF  MERCURY  73 

stance,  quite  often  an  oxide  or  other  compound  of  a  base 
metal.  Various  methods  are  used  to  restore  the  mercury 
to  its  normal  condition.  Cyanide  of  potassium  is  benefi- 
cial through  neutralizing  the  grease,  and  abstracting  the 
oxygen  from  the  oxides.  Likewise  sodium,  by  its  affinity 
for  oxygen,  reduces  the  oxides  of  the  base  metals  which 
are  coating  the  globules  of  mercury,  and  as  sodic  oxide  is 
soluble  in  water,  it  can  be  removed  by  washing,  while  the 
base  metals  enter  the  mercury  forming  amalgam.  After 
treatment  with  these  powerful  alkalies  the  mercury  is 
in  condition  to  do  good  work,  but  owing  to  the  tendency 
of  the  base  metals  to  again  oxidize,  the  relief  is  only 
temporary;  consequently  an  attempt  should  be  made  to 
remove  the  base  metals  entirely.  Retorting  will  best  ac- 
complish this,  if  carried  on  at  moderate  heat,  though  prob- 
ably not  completely,  as  some  of  the  base  metals,  such  as 
lead  and  zinc,  may  distill  over,  particularly  at  high  heat. 
Purification  by  chemical  means  is  easier  and  better.  This 
can  be  accomplished  to  some  extent  by  employing  sul- 
phuric acid;  better  by  hydrochloric,  and  still  better  by 
nitric  acid;  the  acid  being  dilute  in  each  case.  For  this 
purpose  the  mercury  should  be  placed  in  a  glass  or  glazed 
vessel  that  will  not  be  attacked  by  the  acid,  with  a  solu- 
tion of  one  part  nitric  acid  and  from  five  to  ten  parts 
of  water,  and  allowed  to  remain  for  at  least  48  hours  with 
frequent  stirrings  that  all  the  base  metals  may  be  dis- 
solved. Besides  dissolving  the  base  metals  that  contami- 
nate the  mercury,  the  acid  will  also  dissolve  some  of  the 
mercury,  when,  should  it  crystallize,  more  water  and  a 
little  acid  should  be  added  that  the  mercuric  nitrate  may 
be  kept  in  solution.  The  mercury  in  solution  as  mer- 
curic nitrate  will  be  precipitated  when  more  impure  mer- 


74  STAMP  MILLING  AND  AMALGAMATION 

cury  is  added,  by  the  base  metals  replacing  the  mercury 
to  form  nitrates  of  themselves.  By  suspending  a  piece 
of  copper  in  the  solution,  the  mercury  in  solution  can  be 
completely  precipitated.  The  mercury  should  be  washed 
to  remove  all  traces  of  the  acid  before  being  used.  Since 
impure  mercury  does  not  attach  to  gold  with  the  facility 
that  pure  mercury  does,  and  as  it  is  liable  to  become 
'floured'  and  'sickened'  and  thereby  result  in  loss  of  both 
gold  and  mercury,  as  well  as  to  cause  base  metals  to  enter 
the  bullion,  the  purification  of  the  'quick'  should  always 
be  given  the  necessary  attention. 

Mercury  'wets'  those  substances  with  which  it  amalga- 
mates just  as  water  wets  an  ordinary  substance,  forming  a 
thin  film  of  amalgam  about  them  by  the  absorption  of  the 
mercury.  The  surface  tension  of  mercury  is  very  high 
and  its  tendency  is  to  pull  within  itself,  or  below  the  sur- 
face, any  substance  that  amalgamates  with  it ;  in  this  way 
the  particles  of  gold  arrested  on  an  amalgamating  plate 
disappear  below  the  surface.  In  the  case  of  those  substances 
with  which  it  does  not  amalgamate,  its  surface  tension 
acts  negatively  and  it  is  strongly  repellent  to  them.  Gold 
amalgam  containing  90%  of  mercury  is  liquid,  with  87%% 
pasty,  and  at  85%  mercury,  the  amalgam  crystallizes. 

In  crushing  an  ore  for  amalgamation,  the  aim  should 
be  to  crush  just  fine  enough  to  liberate  the  gold  from  its 
matrix  of  quartz  or  other  mineral  that  these  golden  grains, 
or  flakes,  may  be  exposed  to  and  caught  by  the  aid  of  mer- 
cury. Too  coarse  crushing  will  result  in  the  gold  not  be- 
ing liberated,  but  still  enclosed  in  the  gangue  rock  so  that 
the  mercury  is  unable  to  reach  it.  Too  fine  crushing  may 
result  in  beating  the  gold  up  into  flakes  so  fine  that  it  can 
hardly  be  brought  into  contact  with  mercury,  or  may  pos- 


THE  MERCURY  FEED  75 

sibly  coat  it  with  a  film  so  that  it  will  not  readily  amal- 
gamate. 

Mercury  is  fed  into  the  mortar  that  it  may  come  in 
contact  with  the  gold  as  soon  as  liberated  from  the  rock 
by  crushing.  The  action  of  the  stamps  causes  the  mer- 
cury to  become  finely  divided  and  to  be  intermingled 
throughout  the  pulp,  thus  'wetting'  a  considerable  amount 
of  the  gold.  Part  of  this  gold  sinks  about  the  dies  as 
amalgam,  part  is  caught  upon  the  inside  plates,  if  any 
are  used,  and  part  passes  out  through  the  screen.  That 
gold  which  conies  in  contact  with  the  mercury  while  in 
the  mortar  and  which  passes  through  the  screen,  is,  in 
virtue  of  its  being  encased  in  an  envelope  of  mercury, 
larger  than  the  original  native  golden  grain  before  being 
coated  by  the  mercury ;  this,  in  connection  with  the  coat- 
ing of  mercury,  enables  the  lip  or  apron  plate  to  easily 
catch  it.  Gold  that  is  not  so  'wetted'  is  harder  to  catch 
and  may  travel  farther  away  from  the  mortar. 

The  part  of  the  gold  which  is  won  from  the  mortar- 
sand  is  found,  not  in  the  sand  resting  on  the  dies  when 
the  mortar  is  opened,  but  in  that  below  the  level  of  the 
face  of  the  dies.  Where  no  mercury  is  fed  to  the  mortar, 
this  will  be  as  coarse  gold,  but  where  mercury  is  fed  into 
the  mortar,  it  will  be  found  as  amalgam.  This  amount  of 
gold  retained  in  the  mortar-sand  will  vary  with  the  time 
the  gold  has  been  accumulating  in  the  mortar  and  with 
the  height  of  discharge.  A  wide  mortar  with  a  high  dis- 
charge carries  double  or  treble  the  amount  of  pulp  a  nar- 
row mortar,  with  a  low  discharge  will  carry.  In  such  a 
'wave'  mortar  it  will  be  hard  for  the  coarse  gold  and 
amalgam  to  escape  as  their  higher  specific  gravity  will 
cause  them  to  sink  down  through  the  pulp  rather  than  to 


76  STAMP  MILLING  AND  AMALGAMATION 

rise  and  pass  out  through  the  screen,  while  in  the  '  splash ' 
mortar  they  will  be  thrown  through  the  screen  with>  little 
regard  to  their  specific  gravity.  The  amount  of  gold  re- 
tained in  the  mortar  will  also  vary  with  the  size  of  the 
particles  of  gold;  thus,  with  fine  gold  there  will  be  very 
little  caught  in  the  mortar-sand,  while  with  coarse  gold 
the  percentage  will  be  relatively  high. 

The  '  chuck-block '  is  a  piece  of  wood  fastened  to  a  strip 
of  the  same  material,  the  latter  resting  beneath  the  screen 
in  the  screen-frame  slots  of  the  mortar.  Its  purpose  is  to 
fill  a  portion  of  the  surplus  horizontal  space  between  the 
dies  and  mortar  lip  and  screen  frame.  The  chuck-block 
is  sometimes  lined  with  a  copper  amalgamating  plate, 
called  a  'front  inside  plate'  or  'chuck-block  plate.'  Its 
function  is  to  catch  and  hold  upon  its  surface  as  much 
gold  in  the  form  of  a  hard  amalgam  as  possible.  On  ac- 
count of  its  close  proximity  to  the  stamps,  this  plate  is 
subjected  to  much  scouring  action  by  the  pulp,  which  in- 
creases as  the  chuck-block  is  moved  nearer  the  stamps,  or 
the  height  of  discharge  is  lowered,  so  that  mortars  for 
using  inside  plates  are  wide,  14  to  18  inches  across  inside 
of  mortar  at  the  lip. 

The  height  of  the  chuck-block  can  be  varied  by  insert- 
ing beneath  it  and  in  the  screen-frame  slots,  strips  of  wood 
of  varying  thickness,  usually  1  in.,  and  these  may  be  re- 
moved as  the  die  wears  down,  thus  maintaining  the  height 
of  discharge  uniformly  with  reference  to  the  top  of  the 
die.  It  is  not  an  uncommon  practice  to  fasten  securely  a 
strip  of  iron  %  by  %  in.  to  the  face  of  the  chuck-block, 
extending  horizontally  its  entire  length.  Where  the  ore  is 
low  grade  and  the  discharge  low,  amalgam  will  accumu- 


THE  CHUCK-BLOCK  77 

late  along  the  upper  side  of  this  iron  rib,  in  the  angle 
formed  by  it  with  the  copper  plate. 

The  common  form  of  a  curved  chuck-block  is  appar- 
ently wrong,  for  it  is  this  curved  part  or  'belly'  that  is 
in  the  best  position  to  be  scoured.  A  good  method  of 
making  a  chuck-block  is  to  take  a  piece  of  2  by  6-in.  sugar 
pine  and  rip  it  diagonally  across  the  ends,  making  two 
triangular  sectional  lengths.  Attach  one  of  these  to  the 
strip  that  rests  in  the  screen-frame  slot  by  bolts  a  foot 
apart,  using  a  piece  of  strap-iron  in  connection  with  these 
bolts  to  hold  the  lower  end  of  the  plate  in  position  and 
as  a  protection  against  its  being  torn  loose.  Scouring  can 
largely  be  prevented  by  protecting  the  plate  with  a  heavy 
wire  screen  of  quarter  or  half  inch  mesh,  spaced  from  the 
plate  by  a  nut  on  each  bolt  that  attaches  this  screen  to 
the  chuck-block. 

A  copper  plate  is  sometimes  bolted  to  the  back  of  the 
mortar,  this  is  called  a  'back  inside  plate'.  On  account 
of  its  location  it  is  hard  to  handle  and  to  get  at,  and  con- 
sequently is  seldom  used.  These  'inside  plates'  were  for- 
merly used  in  nearly  all  mills,  while  today  they  are  sel- 
dom seen ;  the  tendency  of  modern  milling  is  to  eliminate 
them  entirely.  A  chuck-block  plate  can  be  'run'  in  the 
narrow  mortar,  twelve  inches  wide  at  the  lip,  generally 
used  today,  by  employing  a  sufficiently  high  discharge  and 
considerable  care.  A  back  plate  can  be  used  in  these  mor- 
tars if  it  can  be  set  six  inches  above  the  dies.  It  should  be 
bolted  through  the  mortar  and  have  two  sets  of  bolt  holes, 
that  it  may  be  adjusted  to  the  wear  of  the  dies. 

These  plates  are  usually  of  raw  copper  3/ie  *°  XA  in- 
thick,  that  they  may  not  easily  be  dented  or  worked  out 
of  shape  and  may  stand  the  excessive  wear.  As  they  are 


78  STAMP  MILLING  AND  AMALGAMATION 

cleaned  of  the  amalgam  by  being  chiseled  and  are  liable 
to  be  scoured,  silver-plating  is  an  unnecessary  expense. 
They  are  cleaned  every  10  to  30  days  on  low-grade  ore. 
Where  the  mortar  sand  is  not  removed  at  this  time,  an 
extra  chuck-block  is  kept  on  hand  and  is  merely  substi- 
tuted for  the  enriched  block  after  being  burnished  and 
dressed  with  mercury.  These  plates  must  be  carefully 
watched.  They  are  especially  hard  to  start  and  may  give 
the  amalgamator  some  anxiety  before  a  film  of  amalgam 
is  deposited  over  the  whole  plate.  No  bare  spots  should 
be  allowed  as  they  tend  to  spread. 

These  plates  should  not  be  kept  soft  or  the  amalgam 
will  slough  off,  neither  should  the  amalgam  be  kept  hard 
to  the  point  of  being  brittle,  for  such  amalgam  will  not 
withstand  the  action  of  the  moving  pulp  to  the  extent  that 
a  softer,  more  tenacious,  slightly  yielding  one  will,  nor 
so  readily  catch  the  gold  and  amalgam.  When  using  in- 
side plates,  mercury  is  fed  to  the  mortar  according  to  the 
appearance  of  the  lip  plate  outside  and  an  occasional  ex- 
amination of  the  inside  plates,  usually  that  on  the  chuck- 
block.  This  examination  of  the  chuck-block  can  be  made 
without  removing  the  screen,  by  using  a  canvas  curtain 
instead  of  a  board  to  close  the  opening  in  the  mortar  above 
the  screen;  the  feed  water  is  shut  off  and  as  soon  as  the 
water  is  stamped  out  of  the  mortar,  the  stamps  are  hung 
up ;  a  small  stream  of  water  is  run  along  the  screen  to 
wash  off  the  chuck-block,  when  the  curtain  is  removed 
and  the  plate  inspected.  Should  there  be  a  bare  spot, 
it  is  burnished  and  a  little  mercury  or  soft  amalgam 
is  well  rubbed  in.  After  some  experience  the  amalgama- 
tor is  able  to  roughly  judge  the  condition  of  the  amalgam 
by  reaching  in  with  his  hand  and  feeling  the  amalgam 


INSIDE  PLATES  79 

without  stopping  the  battery.  When  using  inside  plates 
the  greatest  care  should  be  exercised  that  the  feed  is  kept 
just  right  and  that  no  over-feeding  or  choking  of  the 
mortar  occurs.  For  this  purpose  careful  chuck-block  ope- 
rators remove  the  splash  board  from  in  front  of  the  screen, 
that  any  change  in  the  splash  or  wave  of  the  pulp  may  be 
readily  discernible ;  over-feeding  is  indicated  by  a  line  of 
pulp  appearing,  'banking',  at  the  lower  edge  of  the  screen 
and  gradually  rising. 

The  use  of  inside  plates  increases  the  saving  of  gold  in- 
side the  mortar,  by  catching  and  holding  a  large  part  that 
would  otherwise  pass  out  through  the  screen.  This  was 
formerly  considered  desirable  practice,  but  the  use  of 
narrow  mortars,  heavy  stamps,  and  low  discharge  have 
individually  and  collectively  made  good  inside  plate  work 
difficult,  so  that  the  sentiment  is  now  rather  against  their 
use.  The  two  claims  made  in  their  favor  are,  that  they 
should  be  used  when  running  on  high-grade  ore  with  the 
idea  that  the  smaller  the  amount  of  amalgam  to  be  han- 
dled on  the  outside  plates  (referring  especially  to  the 
apron  plates),  the  smaller  the  loss  will  be,  and  that  by 
their  use  fine  or  refractory  gold  can  be  caught  that  can- 
not otherwise  be  saved.  The  arguments  against  their  use 
are,  that  they  necessitate  a  high  discharge  which  cuts 
down  capacity  and  may  cause  overstamping,  and  that 
they  place  the  gold,  or  rather  the  amalgam,  where  it  can 
be  easily  scoured  off  and  lost  should  the  mortar  overfeed. 
On  opening  a  mortar  that  has  been  overfed,  and  has 
run  choked  for  some  time,  the  upper  part  of  the  chuck- 
block  plate  will  usually  be  found  to  be  freshly  scoured, 
sometimes  down  to  the  copper.  This  scoured  amalgam 
being  hard  and  dry,  has  been  found  difficult  to  arrest 


80  STAMP  MILLING  AND  AMALGAMATION 

on  the  plates  unless  they  are  quite  wet  with  mercury, 
consequently  some  of  the  amalgam  passes  beyond 'the 
plates  and  traps  and  is  lost. 

Inside  plates  should  only  be  used  where  it  is  proved 
that  more  gold  can  be  caught  by  their  use  than  without 
them,  and  such  cases  will  be  rare.  In  all  ordinary  cases, 
more  gold  will  be  saved  per  ton,  a  greater  tonnage  crushed 
and  more  satisfaction  in  operating  obtained  without  their 
use.  When  they  are  needed,  it  would  appear  well  to 
employ  both  a  front  and  back  plate  with  the  idea  of  get- 
ting all  the  advantage  there  may  be  in  their  use ;  usually 
there  is  no  back  plate  on  account  of  their  trouble  and  in- 
convenience due  to  their  position;  moreover,  they  cannot 
be  placed  in  some  mortars. 


CHAPTER  VI. 

The  'splash-plate'  is  placed  in  front  of  the  screen,  be- 
ing fastened  to  the  mortar  or  battery  posts,  so  that  the 
pulp  flowing  or  splashing  through  the  screen  may  fall 
or  impinge  upon  it,  run  down  its  surface,  and  drop  on 
the  head  of  the  'lip-plate',  which  is  a  copper  plate  resting 
on  the  lip  of  the  mortar  and  held  in  place  by  the  chuck- 
block,  the  screen  frame,  or  otherwise.  The  advisability 
of  the  splash  plate  is  a  matter  of  individual  opinion  usu- 
ally, and  is  not  used  in  all  cases.  The  conditions  under 
which  work  is  largely  done  today  are  with  a  fine  gold, 
low-grade  ore,  and  a  required  large  capacity,  necessitat- 
ing a  narrow  mortar  and  low  discharge,  which  in  general 
prohibit  the  successful  use  of  inside  plates,  or  of  the 
catching  of  much  gold  in  the  mortar  sand.  The  amalgam 
on  the  splash  and  lip-plates  is  hard,  with  little  tendency 
to  run  and  slough  off,  or  to  granulate  and  break  away, 
consequently  here  is  a  good  place  to  hold  it — a  better 
place  than  on  the  inside,  or  on  apron  plates,  but  not  as 
good  as  in  the  mortar  sand.  Various  forms  of  splash- 
plates  are  in  use.  Besides  that  the  pulp,  after  passing 
through  the  screen,  shall  impinge  upon  this  plate  and 
run  down  to  the  upper  part  of  the  lip-plate,  the  general 
requirement  is  that  they  be  easily  handled  when  neces- 
sary to  remove  them  to  open  the  mortar,  and  that  in  con- 
nection with  the  lip-plate  they  present  a  large  surface. 

The  function  of  the  lip-plate  is  the  same  as  that  of  the 
splash-plate,  to  catch  and  hold  the  particles  of  'wetted' 
gold  and  amalgam  that  have  passed  through  the  screen, 


82  STAMP  MILLING  AND  AMALGAMATION 

and  consequently  their  purpose,  condition,  and  treatment 
is  in  every  way  similar.  Due  to  the  hardness  and  dryness 
of  the  amalgam  on  these  plates,  it  is  supposed  that  but 
little  native  gold,  without  any  wetting  of  mercury  is 
caught  here,  since  there  is  no  surplus  mercury  to  wet  the 
particles.  The  lip  plate  is  as  long  as  the  mortar  lip  on 
which  it  rests  and  ordinarily  six  to  seven  inches  wide, 
which  is  entirely  too  narrow.  Every  mortar  should  be 
so  built  that  a  distributing  or  collecting  box  can  be  fitted 
to  it  at  the  lip.  This  is  an  iron  box  bolted  to  the  mortar 
to  extend  the  lip.  This  gives  a  firm  backing  to  the  lip- 
plate  which  may  be  as  much  as  18  in.  wide.  These  boxes 
distribute  the  pulp  across  the  head  of  the  apron  plates 
through  from  8  to  16  holes,  so  that  by  plugging  up  some 
of  the  holes,  if  necessary,  an  even  distribution  can  be  se- 
cured where  the  discharge  is  not  uniform  across  the  full 
length  of  the  screen.  They  form  a  projection  over  the 
apron-plates,  preventing  leakage;  and  by  not  requiring 
close  contact  with  the  mortar,  prevent  the  apron-table 
from  being"  jarred.  The  lip  and  splash-plates  are  bur- 
nished and  dressed  with  mercury  when  put  in  position 
after  being  cleaned  up.  They  receive  no  further  dress- 
ings unless  their  surfaces  become  fouled  with  some  base 
metal  such  as  zinc,  lead,  or  babbitt  that  has  accidentally 
fallen  into  the  mortar,  or  by  some  compound  in  the  ore 
itself.  In  such  case  the  plates  will  receive  a  stiff  brush- 
ing to  remove  the  foul  film  and  all  loose  material  on  the 
plates  will  be  collected  and  saved  for  treatment  at  clean- 
up time.  The  lip  and  splash-plates  have  their  amalgam 
chiseled  off  semi-monthly  when  running  on  low-grade 
ore.  An  extra  lip-plate  is  kept  on  hand  that  the  battery 
need  not  be  stopped  longer  than  to  make  the  change. 


THE  APRON  PLATE  83 

Like  the  inside  plates,  the  lip  and  splash-plate  need  be  of 
unsilvered  copper  only.  The  lip-plate  is  subjected  to  con- 
siderable abuse  and  should  be  %  in.  thick,  while  %  or  Vie 
is  sufficient  for  the  splash-plates. 

Following  the  lip-plate  and  independent  of  the  mortar 
is  the  'apron-plate',  mounted  on  a  platform  called  a 
'table',  usually  built  of  wood.  This  plate  is  slightly 
wider  than  the  lip-plate  and  from  8  to  28  ft.  long  and 
sometimes  longer,  in  which  case  they  are  built  in  two  or 
more  sections  in  series.  The  apron-plate  is  not  intended 
for  catching  the  bulk  of  the  gold,  except  where  no  mer- 
cury is  fed  to  the  mortar,  but  more  particularly  for  that 
which  has  escaped  being  caught  on  the  plates  above.  Con- 
sequently it  requires  the  greatest  care  and  attention  to 
keep  it  in  good  condition  to  catch  gold.  The  flow  of 
the  pulp  on  the  apron-plate  is  distributed  in  such  manner 
as  to  best  enable  each  particle  to  come  in  contact  with 
the  plate.  Sufficient  mercury  is  kept  on  it  to  render  the 
amalgam  soft  enough  to  wet  any  amalgamable  gold  that 
may  come  in  contact  with  it ;  while  the  amalgam  is  kept 
bright  and  in  an  active  condition  by  frequent  dressings 
of  the  plate. 

It  is  in  the  care  of  the  apron-plate  that  the  amalgamator 
takes  his  greatest  pains  and  pride.  Here  is  found  a  strik- 
ing difference  in  the  ideas  and  methods  of  different  mill- 
men.  Some  keep  the  apron-plate  hard,  even  to  the  verge  of 
the  amalgam  breaking  away,  the  lower  end  of  the  plate  be- 
coming quite  'blue',  for  the  purpose  of  preventing  the  mer- 
cury from  working  down  the  plate  into  the  trap  and  event- 
ually into  the  creek  below.  These  are  said  to  be  amalgam- 
ate 'dry',  as  the  plate  is  kept  comparatively  dry  of  mer- 
cury. Others  keep  the  amalgam  soft  and  plastic,  some- 


84  STAMP  MILLING  AND  AMALGAMATION 

times  overfeeding  mercury  or  dressing  the  plate  so  soft 
that  it  collects  in  globules,  'tears',  which  work  down* the 
plate  carrying  a  little  gold  with  it  into  the  traps  from 
which  a  part  of  it  will  usually  be  lost.  This  is  said  to  be 
'wet'  amalgamation,  as  the  plate  is  kept  wet  or  moist  with 
mercury. 

A  coarse,  easily  amalgamable  gold  will  be  readily 
caught  on  a  hard  plate,  but  fine  gold  requires  a  soft,  plas- 
tic sheet  of  amalgam.  A  hard  plate  is  not  as  active  in 
catching  gold  as  a  soft  one,  since  it  cannot  so  readily 
wet  the  gold,  and  the  surface  tension  in  absorbing  the 
gold  into  and  below  the  surface  of  the  amalgam  is  not  so 
great  on  account  of  the  amalgam  being  crystalline.  The 
coarse  gold,  because  of  its  greater  weight,  sinks  through 
and  is  dragged  along  the  bottom  of  the  film  of  pulp,  com- 
ing in  repeated  contact  with  the  amalgamated  plate  be- 
neath, so  that  with  coarse  gold  this  plate  need  not  be  so 
active  in  wetting  and  catching  it;  whereas  the  fine  gold 
is  carried  along  distributed  throughout  the  pulp  and  only 
occasionally  comes  in  contact  with  the  plate,  consequently 
the  plate  should  be  in  the  best  condition  to  wet  the  gold 
at  the  first  contact.  The  coarse  gold  is  more  liable  to 
be  wetted  in  the  mortar,  and  when  in  such  condition 
should  be  quickly  arrested,  on  even  a  hard  plate.  If  a 
particle  of  hard  amalgam  or  of  gold  is  brushed  over  a 
hard,  dry  amalgamated  surface,  it  will  probably  not  be 
caught,  but  if  brushed  over  a  soft,  wet  spot  it  will  quickly 
attach  itself  to  it.  This  leads  to  the  inference  that  a  wet 
plate  is  the  best  amalgamator.  By  keeping  them  soft  they 
form  the  best  amalgamating  surface  but  if  the  narrow 
margin  of  safety  is  overstepped,  the  water  or  mercury 
will  run  off,  carrying  with  it  the  gold  with  which  it  has 


DRESSING  PLATES  85 

combined  and  herein  lies  the  difficulty  with  wet  amal- 
gamation. Given  an  easily  amalgamable  ore,  the  amal- 
gamator may  approach  dry  amalgamation  and  save  the 
greater  time  and  attention  required  for  wet  amalgama- 
tion without  increasing  loss  in  the  tailing. 

For  cleaning-up  and  dressing  the  apron-plates  amalga- 
mators who  practise  wet  amalgamation  and  employ  con- 
siderable refinement  in  their  methods,  use  a  rag.  They 
remove  part  of  the  amalgam  once  daily  when  treating  an 
average  ore,  so  that  a  constant  amount  remains,  taking 
no  more  off  on  clean-up  day  than  on  any  other  day.  In 
this  'rubbing  up'  some  mercury  is  first  sprayed  on  the 
plates  where  needed.  It  is  well  rubbed  into  the  amal- 
gam, which  is  loosened  and  softened  down  to  the  silvered 
surface.  The  part  of  the  amalgam  to  be  removed  is  pushed 
up  to  the  head  of  the  plate  by  means  of  the  rag,  where  it 
is  removed  with  a  scoop.  The  plate  is  now  well  rubbed 
again,  that  the  amalgam  may  be  softened  down  to  the 
silvered  surface  with  a  view  to  preventing  the  formation 
of  a  hard  film  or  scale  of  amalgam ;  that  the  consistence 
and  texture  of  the  amalgam  may  be  such  as  is  desired, 
that  the  mercury  in  the  amalgam  be  more  securely  held 
by  it,  and  that  the  amalgam  be  worked  into  an  active 
condition.  After  distributing  the  amalgam  evenly  over 
the  plate,  it  is  smoothed  down  at  right  angles  to  the  flow 
of  the  pulp  by  using  a  soft  and  long-straw  whisk-broom. 
This  practice  of  riffling,  or  reaching,  across  the  soft  amal- 
gam has  been  condemned  by  some  as  wrong  for  the  reason 
that  these  riffles  catch  the  fine  iron  and  steel  from  the 
battery  and  sulphide  as  well,  and  thus  coat  or  foul  the 
amalgamating  surface.  The  fact  that  these  tiny  grooves 
do  act  thus,  speaks  well  for  their  function  in  catching 


86  STAMP  MILLING  AND  AMALGAMATION 

gold.  However,  these  riffles  can  be  avoided  by  smoothing 
the  surface  of  the  amalgam  with  a  fine-haired  paint  or 
calcamining  brush.  An  experiment  should  be  conducted 
by  brushing  one  side  of  a  plate  crosswise  and  the  other 
lengthwise  to  the  flow  of  the  pulp.  Care  is  used  that  all 
the  particles  of  amalgam  are  well  set,  for  this  reason 
many  amalgamators  finally  run  the  brush  up  and  down 
on  each  side  of  the  table  where  there  is  liable  to  be  loose 
amalgam  in  the  corners. 

They  keep  a  good  bed  of  amalgam  on  the  first  section 
of  the  apron-plate  and  less  on  the  following  plates.  This 
amalgam,  especially  that  on  the  first  plate,  should  be  of 
such  consistence  that  it  can  be  pushed  up  with  the  finger 
and  remain  without  flattening  out ;  that  it  be  soft  and 
plastic  like  putty ;  that  the  brush  lines  do  not  run  or  dis- 
appear; that  it  does  not  run  or  slough  down  the  plate, 
as  indicated  by  its  being  caught  lower  down  than  usual 
and  piling  up  at  the  drops  between  plates,  and  that  no 
tears  of  mercury  appear  or  hang  to  the  edges  of  the  in- 
dividual plates.  The  idea  is  to  keep  the  plate  as  wet  as 
possible  without  losing  any  of  the  mercury  or  amalgam. 
The  loss  of  mercury  carrying  away  gold  is  the  main  ar- 
gument against  wet  amalgamation  and  the  greatest  care 
and  study  should  be  given  to  prevent  any  abnormal  loss 
and  yet  keep  the  plate  moist.  In  appearance  this  amal- 
gam should  look  neither  hard  and  dead,  nor  like  a  mir- 
ror, but  have  a  white  frosted  appearance.  It  has  been 
said  that  the  plate  should  have  the  appearance  of  a  silver 
dollar;  this  is  correct  if  the  kind  of  a  dollar  is  stated. 
The  peculiar  whitish  lustre  of  a  newly  minted  silver  coin 
is  the  appearance  the  amalgam  should  have;  but  should 
the  amalgam  appear  like  the  ordinary  silver  coin  that  has 


CONSISTENCE  OF  AMALGAM  87 

lost  its  'youthful  bloom',  it  is  too  hard,  too  dead,  too  crys- 
talline for  the  good  amalgamation  of  gold  that  is  at  all 
difficult  to  catch. 

The  apron-plate  is  examined  at  its  head  at  intervals 
of  one-half  to  two  hours,  usually  hourly,  clearing  it  by 
means  of  a  stream  of  water  for  the  purpose  of  learning 
how  amalgamation  is  progressing  and  how  much  mercury 
must  be  fed  into  the  mortar.  At  first  the  amalgamator 
will  press  or  mark  the  amalgam  with  his  finger  to  learn 
its  consistence,  but  in  a  short  time  will  be  able  to  tell  by 
its  appearance  alone.  After  examining  the  plate,  mer- 
cury is  fed  into  the  mortar;  the  amount  being  judged 
by  the  appearance  of  the  plate.  It  should  be  sufficient  to 
keep  the  plate  in  good  condition  until  the  next  examina- 
tion. Should  the  plate  upon  this  inspection  have  its  amal- 
gam of  the  proper  consistence  and  be  wet  with  mercury 
to  the  proper  point,  a  normal  amount  of  mercury,  as  indi- 
cated by  experience  with  that  ore,  is  fed  to  the  mortar; 
should  the  amalgam  appear  hard  and  dry,  an  extra 
amount  of  mercury  is  fed;  should  the  plate  appear  too 
soft  and  wet,  no  mercury  is  added  that  it  may  harden 
to  the  right  condition  by  the  time  of  the  next  examina- 
tion. The  mercury  is  weighed  each  morning  to  ascertain 
the  amount  fed  to  the  mortars  the  previous  day.  After 
the  plates  have  become  saturated  with  mercury  and 
loaded  with  what  may  be  termed  their  constant  of  amal- 
gam, the  amalgamator  divides  the  ounces  of  bullion  re- 
covered during  a  stated  period  by  the  number  of  ounces 
of  mercury  fed  to  the  mortars  during  that  time  and  the 
result  becomes  a  factor  by  which  he  estimates  the  amount 
of  gold  amalgamated  each  day  and  from  this  what  the 
month's  run  should  be.  With  an  amalgamator  who  uses 


88  STAMP  MILLING  AND  AMALGAMATION 

care  in  feeding  mercury  and  a  gold  that  does  not  vary 
greatly  in  the  size  of  its  particles,  this  is  a  fairly  accurate 
factor  and  a  satisfactory  method,  being  equal  to  the  hap- 
hazard methods  of  sampling,  assaying,  and  estimating  that 
obtain  in  many  mills. 

While  it  is  well  to  inform  the  amalgamator  whether 
low,  medium,  or  high-grade  ore  is  being  put  into  the  mill, 
he  makes  but  little  use  of  this  information  except  in  cor- 
roboration  as  it  is  so  often  unreliable  and  he  is  also 
unable  to  know  when  this  ore  of  a  different  nature  is 
going  to  reach  the  mortars.  At  the  hourly  examination  of 
the  plates,  he  is  able  to  see"  how  much  amalgam  is  being 
deposited  or  built  up  on  the  plates,  and  this  is  a  true  in- 
dex of  the  gold  content  of  the  ore,  unless  it  has  suddenly 
become  base  or  unamalgamable.  When  mercury  is  fed 
in  the  right  quantity  and  at  the  proper  intervals,  very  lit- 
tle additional  mercury  is  required  in  dressing  the  plates. 
Where  inside  plates  are  used,  the  lip  is  used  as  the  out- 
side indicator  of  the  amount  of  mercury  to  be  fed  to  the 
mortar  to  keep  the  inside  plates  in  condition;  should  the 
apron-plate  become  dry,  a  little  mercury  can  be  sprayed 
on  the  dry  spots  at  the  head  of  the  table  from  a  bottle 
or  bag  without  stopping  the  battery.  If  one  side  of  the 
plate  is  dry  and  the  mercury  is  fed  to  that  end  of  the 
mortar,  the  larger  part  will  come  out  on  that  side  if  the 
mortar  is  of  the  splash  or  low-discharge  type. 

Amalgam,  or  mercury  containing  gold,  has  a  greater 
affinity  for  gold  than  mercury  alone.  Mercury  that  has 
the  gold  only  partly  strained  out  of  it  is  better  for  amal- 
gamating than  gold-free  mercury.  A  bed  of  gold  amal- 
gam is  a  most  active  catcher  of  gold — superior  to  silver 
amalgam,  and  the  plates  should  always  be  covered  with 


TREATMENT  OF  PLATES  89 

it.  Amalgam  acts  tenaciously  in  holding  the  mercury  on 
the  plates.  New  plates  dressed  with  mercury  do  not  give 
the  best  results  until  a  bed  of  amalgam  has  spread  over 
them,  as  has  been  proved  by  sampling  and  assaying  the 
pulp  after  passing  over  plates  when  in  this  condition ; 
consequently  close  clean-ups  with  chisels  should  be 
avoided  unless  some  of  the  amalgam  is  returned  to  the 
plates.  Such  clean-ups,  however,  in  connection  with  thor- 
oughly rubbing  and  softening  the  amalgam,  prevents  the 
formation  of  hard  layers  of  amalgam,  but  the  close  *  skin- 
ning' of  the  plate  is  detrimental  to  the  silver-plating,  and 
some  amalgamators  will  not  use  even  a  rubber  for  remov- 
ing the  amalgam.  A  rag  requires  more  labor  to  use  and 
is  harder  on  the  hands  than  a  brush,  so  that  whisk  brooms 
are  commonly  used  in  dressing  the  plates,  the  soft  amal- 
gam being  pushed  to  one  spot  by  a  worn  down  or  cut  off 
whisk.  The  last  plate,  or  plates,  are  kept  free  from 
soft  amalgam,  the  object  being  to  run  them  harder,  in 
which  condition  they  are  better  able  to  retain  the  mer- 
cury and  soft  amalgam  that  may  run  down  from  the  upper 
plates. 

Where  'dry'  amalgamation  is  practised,  the  procedure 
does  not  differ  materially.  In  dressing  the  plates,  no  ef- 
forts are  made  to  soften  the  bottom  part  of  the  amalgam 
or  to  keep  it  soft  in  comparison  with  the  silver-plated  sur- 
face; the  efforts  being  confined  to  putting  the  surface 
of  the  amalgam  in  good  condition  for  catching  the  gold. 
Any  soft  amalgam  that  may  be  considered  as  surplus  is 
removed,  or  only  that  which  loosens  and  breaks  away  in 
the  dressing,  the  'crumbs',  is  removed;  the  balance  be- 
ing allowed  to  remain  and  become  hard  until  the  monthly 
or  semi-monthly  clean-up,  when  it  is  removed  by  being 


90  STAMP  MILLING  AND  AMALGAMATION 

chiseled  and  scraped  off,  care  being  used  not  to  reach 
into  the  silver-plating.  After  the  chiseling,  mercury  is 
rubbed  in  to  soften  the  remaining  amalgam,  which  is 
scraped  together  by  a  'rubber' — a  piece  of  pure  rubber 
or  rubber  belting.  At  many  of  the  mills  where  dry  amal- 
gamation is  in  vogue,  the  plates  are  *  sweated'  at  inter- 
vals by  pouring  boiling  water  on  them,  or  by  placing  a 
cover  of  wood  or  blankets  over  them  and  turning  steam 
or  boiling  water  in  underneath.  This  results  in  loosening 
the  film  of  hard  amalgam  that  will  always  form,  especially 
when  amalgamating  dry,  so  that  it  may  be  scraped  and 
scaled  off.  A  large  amount  of  gold  that  would  be  other- 
wise locked  up  on  the  plates  is  secured  by  'sweating,' 
but  it  is  destructive  to  the  silver-plating  and  dangerous 
on  account  of  the  possibility  of  salivating  the  workmen. 
The  paternal  laws  of  Australia  prohibit  approaching  a 
'sweated'  plate  until  after  the  lapse  of  a  certain  length 
of  time,  which  makes  the  '  sweat '  of  little  avail. 

As  to  the  relative  merits  of  wet  and  dry  amalgamation : 
Dry  amalgamation  answers  where  the  gold  is  coarse  and 
easily  amalgamated,  and  as  it  requires  less  labor  and  at- 
tention, and  usually  entails  a  smaller  loss  of  mercury,  it 
may  be  advisable  in  some  cases.  As  the  gold  becomes  finer 
and  more  difficultly  amalgamable,  the  necessity  of  amal- 
gamating wet  increases.  The  life  and  good  condition  of 
the  plates  is  increased  by  amalgamating  under  the  wet 
conditions  described.  Where  amalgamation  is  done  alto- 
gether outside,  the  plates  should  be  kept  wet,  certainly 
wetter  than  is  usual  with  dry  amalgamation,  since  the 
gold,  not  being  wetted  in  the  mortar,  will  have  a  tendency 
to  slip  farther  over  the  dry  surface. 

For  amalgamating  on  the  apron  plate,  the  feed  water 


THE  GRADE  OF  PLATES  91 

used  in  the  mortar  should  be  just  sufficient  to  carry  the 
pulp  down  the  table  in  waves  that  appear  retarded,  almost 
but  never  stopping,  rolling  over  and  over  and  breaking 
up,  that  each  part  and  particle  of  the  pulp  may  be  brought 
in  contact  with  and  dragged  over  the  amalgamated  sur- 
face of  the  plate  as  much  as  possible.  The  coarse  gold 
sinks  to  the  bottom  of  the  pulp  and  is  caught,  usually  on 
the  upper  section  of  the  plate,  within  2  or  3  ft.  of  the 
mortar  lip,  while  the  fine  gold  is  carried  along  by  the 
sweep  and  rush  of  the  pulp,  and  being  unable  to  sink 
through  it,  must  wait  until  the  turning  over  and  break- 
ing up  of  the  pulp  finally  brings  it  in  contact  with  the 
plate.  A  difference  has  been  noticed  in  the  proportion 
of  the  gold  to  the  silver  in  amalgam  caught  at  the  head 
of  the  table  as  against  that  caught  at  the  foot.  This  has 
been  credited  to  the  greater  ease  with  which  gold  is  amal- 
gamated than  silver,  but  a  sizing  of  the  gold  particles 
in  the  ore  might  reveal  that  the  fine  gold  caught  farther 
away  from  the  mortar  contains  a  higher  proportion  of 
silver. 

Where  difficulty  is  experienced  in  amalgamating  the 
gold,  or  where  the  plates  appear  to  be  too  short,  less  water 
should  be  used  in  the  mortar,  even  if  the  grade  of  the 
plates  has  to  be  increased,  that  the  pulp  may  be  dragged 
and  rolled  over  the  plates  rather  than  sluiced.  The  fall 
of  plates  now  in  use  varies  from  l1/^  to  3  in.  per  foot,  and 
should  not  be  less  than  2  or  2%  in.  A  low  fall  requires 
too  much  water  in  the  mortar  to  keep  the  pulp  from  bank- 
ing on  the  plates,  to  give  good  apron-plate  amalgamation, 
especially  if  the  ore  contains  much  sulphide  or  other  heavy 
material.  It  is  better  to  make  the  grade  too  great  than 
too  little.  There  are  various  methods  of  constructing 


92  STAMP  MILLING  AND  AMALGAMATION 

plate  tables  that  allow  the  grade  to  be  easily  changed, 
and  it  is  well  to  use  such  construction.  Where  the*  pulp 
has  banked  on  the  plate,  the  careful  amalgamator  does 
not  hose  it  off  with  a  large  volume  of  water,  thereby  los- 
ing any  gold  caught  in  the  pulp  or  only  lightly  attached 
to  the  plate,  or  any  spikes  of  crystalline  amalgam ;  he  turns 
on  a  light  stream-  and  slowly  and  gently  washes  the  de- 
posit away.  Some  millmen  place  a  stick  of  wood  diagon- 
ally on  the  plate  above  a  bank  of  sand,  diverting  the 
stream  of  pulp,  and  thus  washing  the  sand  away. 

It  was  customary  years  ago  to  use  a  spray  of  clear  water 
from  a  perforated  pipe  or  distributing  box  at  the  head  of 
the  table  tQ  slightly  retard  the  pulp  and  to  cause  the  gold 
to  settle  and  attach  itself  to  the  plate.  The  crushing  ca- 
pacity at  that  time  was  small,  due  to  light  stamps  and 
wide  mortars,  but  with  the  reverse  of  these  conditions 
now,  all  the  water  that  it  is  safe  to  use  should  be  intro- 
duced through  the  mortar  in  the  effort  to  pass  the  mate- 
rial through  the  screen  as  fast  as  possible.  Wherever  ex- 
traordinarily long  plates  are  required,  it  will  be  found 
that  an  excess  of  water  is  being  used,  or  that  the  gold  is 
extremely  fine.  Where  extra  plates  are  needed  or  much 
amalgam  is  caught  on  the  last  plate  due  to  an  excess  of 
water,  the  pulp  should  be  divided  between  two  short  extra 
tables  rather  than  one  long  extra  table,  so  as  to  induce 
the  rolling  of  the  pulp  so  necessary  to  good  plate  amalga- 
mation. Where  the  plates  are  so  long  that  the  lower  plate 
scours,  and  much  mercury  is  required  to  keep  it  in  con- 
dition, while  it  returns  no  amalgam,  the  plates  may  be 
shortened,  or  the  use  of  more  water  in  the  mortar  can  be 
tried.  This  last  should  cause  the  gold  to  be  caught  lower 
down  and  the  lower  plates  to  keep  in  condition  while  the 


CARE  OF  PLATES  93 

amalgamator  can  salve  his  conscience  for  this  departure 
from  good  amalgamating  practice  by  the  increased  ton- 
nage due  to  the  use  of  a  larger  quantity  of  water. 

A  plate  requires  the  constant  addition  of  mercury  and 
gold  to  keep  it  from  being  denuded  by  the  pulp.  Running 
on  dumps  of  extremely  low-grade  ore,  as  is  so  often  done 
before  the  final  shutting  down  of  a  mill,  has  been  unsat- 
isfactory in  many  instances  for  the  above  reason.  Also, 
coarse  crushing  is  generally  resorted  to  in  the  -effort  to 
compensate  for  the  low  value  of  the  rock  thereby  increas- 
ing tonnage,  whereas  the  gold  is  usually  finer  and  may 
require  finer  crushing  to  liberate  it.  Should  the  plates 
scour  when  running  on  this  low-grade  rock,  a  small 
amount  of  water  should  be  used  in  the  mortar  to  induce 
the  gold  to  be  amalgamated  on  a  short  length  of  plate, 
while  removing  the  lip  and  splash  plates  may  assist  in 
keeping  the  apron  plates  in  condition.  Clear  water  will 
carry  off  mercury  and  amalgam  and  when  allowed  to  run 
over  a  plate  for  some  time,  the  amount  should  be  reduced 
to  just  sufficient  to  keep  them  wet  and  prevent  oxidation 
and  discoloration. 

At  many  mills  running  on  low-grade  ore,  or  where  the 
value  is  largely  in  the  sulphide,  it  is  customary  to  clean 
and  dress  the  plates  without  stopping  the  battery,  thus 
saving  the  considerable  labor  involved  in  hanging  up  the 
stamps  and  the  loss  of  duty  in  stopping  the  battery.  This 
can  be  easily  and  satisfactorily  accomplished  by  dividing 
the  plate  into  two  sections  by  placing  a  strip  of  wood 
permanently  down  the  center,  and  directing  the  entire 
flow  to  either  half  of  the  plate  by  plugging  the  holes  on 
one-half  of  the  distributing  box,  usually  also  cutting  down 
the  amount  of  water  entering  the  mortar.  If  no  distrib- 


94  STAMP  MILLING  AND  AMALGAMATION 

uting  box  can  be  bolted  to  the  mortar,  the  plate  table  can 
be  built  so  that  a  wooden  trough  distributer  can  fre  set 
beneath  the  lip  of  the  mortar.  At  the  Empire  mill,  Grass 
Valley,  California,  the  tables  are  so  fitted  to  the  mortars 
that  when  dressing  a  plate,  a  launder  of  sufficient  size  and 
length  to  carry  the  flowing  pulp  to  the  plate  of  an  adjoin- 
ing battery  is  inserted  under  the  mortar  lip.  At  first 
sight  it  would  appear  that  overloading  a  plate  in  this 
manner  would  be  bad  practice,  but  the  actual  experience 
has  been  so  satisfactory  that  experiments  by  sampling  and 
assaying  the  tailing  under  the  different  conditions  should 
be  made  before  condemning  this  practice  at  any  mill. 
Mills  using  these  systems  generally  have  long  plates.  In 
view  of  the  Empire  mill  practice,  it  should  be  possible  to 
deflect  the  pulp  to  a  launder  or  pipe  leading  to  an  extra 
plate  set  over  the  concentrating  floor  or  at  the  side  of  the 
mill. 

The  plate  table  should  be  as  free  from  jar  as  possible, 
preferably  carried  up  from  the  ground  independent  of 
the  flooring  and  framework  of  the  mill.  It  has  been 
claimed  that  the  jar  is  beneficial  in  promoting  amalgama- 
tion and  attention  has  been  called  to  the  excellent  work 
of  shaking  plates  mounted  on  vanner  concentrators  after 
removing  the  belt  and  following  the  apron  plates.  The 
motion  these  shaking  plates  receive  is  an  even,  gentle,  van- 
ning motion  like  that  which  causes  the  gold  to  settle  in 
the  gold  pan ;  while  the  vibration  to  which  a  plate  table  is 
subjected  from  coming  in  contact  with  the  mortar,  or 
from  the  jar  of  the  floor,  in  a  poorly  built  mill,  is  quite 
different — one  that  causes  the  mercury  to  exude  from  the 
amalgam  in  globules,  and  the  amalgam  to  granulate. 

Plate  tables  are  commonly  built  of  1%  and  2  in.  plank, 


DROPS  IN  THE  PLATES  95 

laid  either  lengthwise  or  transversely,  with  side  pieces  of 
the  same  material  10  to  12  in.  wide.  A  better  method  is 
to  use  2  by  4,  4  by  4,  or  3  by  5  in.  planed,  well-seasoned 
lumber,  placed  lengthwise  of  the  table,  either  spiked  to- 
gether or  bolted  across  the  width  of  the  table  every  three 
feet,  and  put  together  with  a  thick  waterproof  paint. 
With  such  a  table  there  should  be  no  leakage.  When  drops 
are  introduced  they  are  made  shallow,  the  material  be- 
ing cut  out  as  required.  The  supports  beneath  should  be 
as  simple  as  possible,  except  where  rendered  complicated 
by  introduction  of  means  for  changing  the  grade.  The 
table  should  be  dressed  down  to  make  the  center  1/6  in. 
lower  than  the  edges,  or  the  pulp  will  riffle  toward  each 
side  of  the  table  forming  washes  there  and  leaving  it  too 
shallow  in  the  center. 

Drops  between  the  plates  are  introduced  as  the  gold 
tends  to  collect  where  the  pulp  strikes  the  plate ;  also  the 
farther  the  pulp  travels  down  a  straight  plate,  the  greater 
its  speed  becomes,  one  of  the  visible  indications  of  which 
is  the  increased  size  of  the  waves  at  the  lower  end  of  the 
table.  A  drop  serves  to  start  the  pulp  off  anew  and  pre- 
vent the  acceleration  of  the  speed  of  the  pulp,  wherefore 
it  is  preferable  to  decrease  the  grade  of  the  lower  end  of 
the  table;  it  also  breaks  the  flow  of  the  waves,  thereby 
giving  the  fine  float  and  suspended  gold  a  better  oppor- 
tunity to  come  in  contact  with  the  plate,  and  also  induces 
a  more  even  distribution  of  the  pulp.  As  the  amalgam 
piles  up  at  these  drops,  they  are  much  in  favor;  the  gen- 
eral idea  being  that  by  increasing  the  number  of  drops, 
the-  length  of  the  plates  may  be  diminished.  This  idea  is 
not  entirely  correct  for  the  amount  of  amalgam  collected 
at  these  drops  will  depend  upon  how  the  amalgamation 


96  STAMP  MILLING  AND  AMALGAMATION 

is  being  conducted.  Where  too  much  mercury  has  been 
used  in  dressing  the  plates  or  fed  through  the  mortar, 
the  amalgam  will  run  and  slough  off  and  collect  at  these 
drops,  so  that  the  amalgamator  when  examining  the 
plates,  should  also  use  the  drops  as  an  indicator.  About 
the  only  objection  to  drops  is  that  they  interfere  to  a 
slight  extent  with  the  quick  dressing  of  the  plates.  A  drop 
of  one-half  inch  is  sufficient  to  give  the  desired  results, 
and  more  than  three-quarters  of  an  inch  is  liable  to  cause 
scouring.  Where  a  drop  scours,  a  strip  of  wood  should 
be  used  as  a  baffle-board  to  ease  the  pulp  to  the  plate. 

Blankets  can  be  placed  underneath  the  plates  if  neces- 
sary to  even  up  the  table.  The  sides  of  the  plates  should 
be  turned  up  and  the  ends  slipped  well  under  one  another 
that  neither  the  water  nor  the  mercury  may  work  through 
to  the  floor.  It  is  preferable  to  have  the  plates  held  down 
by  side  strips  and  overlapping  without  the  use  of  screws. 
Designing  the  tables  so  that  the  plate  adjacent  to  the  mor- 
tar may  be  rolled  away  from  the  mortar  and  over  the  re- 
maining part  of  the  table,  or  that  the  entire  table  may  be 
moved  forward  is  an  unnecessary  expense,  as  advantage 
of  this  is  only  taken  when  removing  a  mortar  or  repair- 
ing a  mortar  block. 

At  one  time  sluice-plates  were  in  vogue.  Following  the 
lip-plate  and  mortar  was  one  apron-plate  the  width  of 
the  mortar  and  4  ft.  long.  The  plate  table  was  then  nar- 
rowed down  to  hold  three  or  four  plates  of  half  the  width 
of  the  apron-plate.  Only  the  heavier  gold  and  amalgam 
could  sink  down  through  the  flood  to  which  the  pulp  was 
contracted  while  running  over  these  plates,  also  the  ten- 
dency of  the  pulp  to  scour  the  plates  was  increased.  These 


MERCURY  TRAPS  97 

sluice-plates  were  extensively  used  at  one  time,  but  have 
been  entirely  discarded. 

The  pulp  falling  from  the  last  apron-plate  is  collected 
by  a  'tail-box'  delivering  to  a  mercury  trap  or  launder. 
The  tail-box  should  be  fitted  with  what  is  known  as  a 
'treasure-box.'  This  is  simply  another  compartment  in 
addition  to  the  one  collecting  the  pulp  for  the  launder; 
a  swinging  door,  or  lid,  enables  the  pulp  from  the  plates 
to  be  directed  to  either  compartment.  The  plate  is  washed 
down  with  a  heavy  stream  of  water,  preparatory  to  dress- 
ing, into  this  second  compartment;  after  dressing,  the 
plate  is  again  washed  down,  this  time  with  a  light  stream. 
The  function  of  the  treasure-box  is  to  catch  the  particles 
of  loose  amalgam  that  might  otherwise  be  lost,  and  also 
the  rich  sulphides  that  have  attached  themselves  to  the 
plate  by  reason  of  the  amalgamating  of  an  exposed  face 
of  contained  gold.  This  box  is  very  necessary  when  run- 
ning on  rich  ore.  A  portable  trough  that  can  be  set  be- 
neath the  lower  edge  of  the  plate  is  used  to  serve  the 
same  purpose.  The  treasure-box  is  a  handy  accessory 
when  the  mortar  is  opened,  as  the  pulp  and  rock  that  al- 
ways litter  the  mortar  lip  and  upper  apron  plate  at  such 
times  is  sluiced  into  the  box  to  be  returned  to  the  mortar 
without  getting  into  the  traps  or  on  the  concentrators. 

A  plate  should  be  tried  in  the  tail-box,  in  the  launders, 
and  on  the  distributing  box  of  the  concentrators,  but  these 
plates  must  be  dressed  at  intervals  and  kept  in  good  con- 
dition to  enable  them  to  catch  anything.  Riffles  should 
be  placed  in  the  launders  or  made  by  notching  the  bot- 
tom plank  of  the  tailing  flume,  as  even  with  the  most 
careful  amalgamating  a  little  amalgam  escapes  which 
can  be  occasionally  gathered  from  them  without  expense. 


98  STAMP  MILLING  AND  AMALGAMATION 

Patent  amalgamators  placed  following  the  apron-plate 
have  been  successful  in  many  instances,  but  it  does  not 
appear  wherein  they  are  superior  to  plates  except*  that 
they  may  present  a  larger  amalgamating  surface,  op- 
posed to  which  is  the  more  refined  manipulation  that  can 
be  practised  on  the  ordinary  plate.  It  is  better  to  make 
a  trial  of  them  rather  than  an  outright  purchase.  Too 
often  such  machines  do  not  get  a  fair  test,  due  to  the  in- 
experience of  the  millman  and  to  his  proceeding  on  the 
theory  that  their  introduction  in  the  mill  is  a  reflection 
on  his  ability  as  an  amalgamator,  as,  in  truth,  it  often  is. 

Where  the  mercury  is  fed  inside  the  mortar  with  the 
object  that  the  particles  of  gold  may  be  wetted  and  that 
sufficient  mercury  may  come  through  the  screen  in  the 
form  of  amalgam,  or  in  a  finely  divided  state,  to  keep  the 
'  outside '  plates  in  proper  condition,  they  are  said  to  prac- 
tise '  inside  amalgamation, '  though  formerly  this  term  had 
more  particular  reference  to  catching  and  holding  the 
gold  on  the  inside  of  the  mortar.  Where  no  mercury  is 
fed  to  the  mortar,  but  all  is  added  on  the  outside  plates, 
and  the  gold  comes  through  the  screens  as  native  gold  un- 
wetted  by  mercury,  they  are  said  to  practise  '  outside  amal- 
gamation. ' 

Outside  amalgamation  involves  no  radical  departures 
from  the  general  methods  of  plate  amalgamation,  when 
mercury  is  fed  to  the  mortar.  Lip  and  splash-plates  are 
dispensed  with,  all  the  gold  being  caught  on  the  apron- 
plates.  A  common  fault,  where  outside  amalgamation  is 
practised,  is  that  the  plates  are  dressed  too  wet  and  then 
allowed  to  become  too  dry  before  dressing  again.  This 
can  be  avoided  and  the  plate  kept  in  condition  by  sprink- 
ling a  little  mercury  from  a  bag  on  the  dry  spots  at  the 


OUTSIDE  AND  INSIDE  AMALGAMATION  99 

head  of  the  table  without  stopping  the  battery,  but  it  must 
be  carefully  done.  Of  coarse  gold  the  greater  part  can 
often  be  caught  on  the  first  12  in.  of  the  plate  surface  if 
mercury  is  dropped  on  as  required.  When  mercury  is 
not  fed  as  needed  and  the  plate  becomes  hard  and  coated, 
as  quickly  happens  when  treating  rich  ore,  the  gold  will 
tend  to  slip  over  the  dry  part  and  be  caught  lower  down. 
With  outside  amalgamation  the  plates  will  usually  be 
dressed  from  two  to  three  times  as  often  as  when  the 
mercury  is  fed  through  the  mortar.  Outside  amalgama- 
tion is  interesting  and  affords  excellent  opportunity  to 
study  plate-work;  some  experience  with  it  will  cause  the 
amalgamator  to  incline  toward  wet  amaglamation. 

As  to  whether  outside  or  inside  amalgamation  should 
be  the  practice  is  properly  dependent  on  the  ore  and  the 
crushing  machinery,  as  indicated  by  the  amount  of  mer- 
cury lost,  the  increase  or  decrease  in  the  amount  of  gold 
saved,  and  the  ease  with  which  amalgamation  can  be  con- 
ducted; but  in  most  cases  the  prevailing  local  custom  is 
followed  without  testing  the  merits  of  the  other  method. 
The  stamping  of  the  mercury  in  the  mortar  comminutes 
it  into  fine  globules;  moreover,  the  tendency  of  mercury 
in  the  presence  of  water  and  agitation  is  to  form  small 
globules  to  some  extent  which  may  be  carried  away  me- 
chanically in  a  finely  divided  condition.  This  natural  ten- 
dency of  the  mercury  to  'flour'  is  increased  when  it  is 
impure,  or  its  association  with  deleterious  substances  in 
the  ore,  so  that  great  loss  may  result  from  either  of  these 
causes.  With  a  clean  ore,  the  loss  of  mercury  in  the  prac- 
tice of  inside  amalgamation  is  usually  about  double  that 
occurring  when  doing  outside  work,  and  with  ores  con- 
taining substances  that  are  detrimental  to  the  mercury, 


100  STAMP  MILLING  AND  AMALGAMATION 

the  loss  may  be  five  or  six  times  as  great.  It  is  impossible 
to  say  how  much  gold  is  carried  off  by  this  vagrant  mer- 
cury, but  it  should  be  less  than  that  indicated  by  assaying 
the  mercury  caught  in  a  newly  cleaned  mercury  trap,  as 
the  larger  part  must  be  lost  in  the  form  of  a  finely  divided 
mercury  that  has  amalgamated  with  but  little  gold.  Oil 
and  grease,  talcose  and  clayey  ores,  arsenic  and  the  com- 
pounds of  arsenic  and  antimony  are  the  worst  enemies  to 
amalgamation — coating  the  floured  mercury  with  a  film 
so  that  is  becomes  permanently  floured,  in  which  condi- 
tion it  is  said  to  be  'sickened.'  Oil  and  grease  are  par- 
ticularly bad  and  every  effort  should  be  made  to  prevent 
them  from  coming  in  contact  with  the  ore,  or  mortar,  or 
the  plates. 

Amalgamation  in  cyanide  solution  presents  no  difficul- 
ties within  the  requirements  made  on  it — a  close  saving  on 
the  plates  not  being  essential.  The  loss  of  mercury  is 
high  as  it  is  dissolved  to  be  precipitated  in  the  zinc  boxes. 
This  deposition  on  the  zinc  appears  to  assist  the  precipi- 
tation of  the  gold 'and  silver,  due  to  the  formation  of  a 
galvanic  couple.  The  mercury  cannot  be  recovered  econ- 
omically under  ordinary  conditions.  The  strength  of  the 
solution  should  be  kept  down  to  one-half  pound  of  potas- 
sium cyanide  per  ton  of  solution  to  prevent  the  too  rapid 
dissolution  of  the  plates  and  mercury,  though  a  solution 
of  two-pound  strength  has  been  successfully  used.  As 
the  life  of  the  plates  is  limited  from  six  to  nine  months 
and  the  amalgamation  rather  roughly  carried  on,  silver- 
plating  the  plates  is  an  inadvisable  expense.  No  aids  or 
methods  of  prolonging  the  life  of  these  plates  has  yet  come 
into  use,  notwithstanding  the  item  for  renewals  is  an  im- 
portant one.  The  lower  plate  and  lower  part  of  each 


CORROSION  BY  CYANIDE  101 

plate  is  corroded  first.  As  the  amalgam  is  cleaned  up  the 
closest  from  these  places,  it  gives  weight  to  the  natural 
conclusion  that  a  thick  coating  of  hard  amalgam  would 
prolong  the  life  of  the  plates.  The  cyanide  keeps  the 
plates  beautifully  free  from  stains  as  it  dissolves  the  cop- 
per compounds  as  fast  as  formed,  and  owing  to  its  low 
strength  does  not  harden  the  plates  to  the  extent  that 
might  be  expected.  The  plate  tables  should  be  built  water 
tight  that  neither  the  mercury  nor  solution  may  run 
through,  for  the  plates  are  corroded  irregularly  and  it 
may  not  be  convenient  to  remove  them  when  the  first  spot 
appears.  Iron  tables  or  those  having  the  bed  of  plate- 
iron  or  steel  and  water  tight  would  be  excellent.  Raw 
copper  plates  of  extra  thickness  with  backs  covered  with 
a  thoroughly  solution-proof  paint,  in  two-foot  sections 
with  a  drop  between  each,  the  sections  to  be  easily  and  in- 
dependently removable  or  changeable,  are  the  lines  along 
which  these  tables  should  be  designed.  The  plates  should 
not  be  allowed  to  project  beyond  their  backing  as  the 
ends  are  gradually  dissolved  down  to  dangerous  knife 
edges.  The  solution  being  weak  does  not  injure  the  hands 
of  the  workmen,  though  it  may  make  them  rough  at  times. 
Rubber  gloves  are  not  required.  In  designing  and  build- 
ing a  mill  for  crushing  in  cyanide  solution,  every  effort 
should  be  taken  to  prevent  the  leaking  and  spilling  of  the 
gold-bearing  solution,  while  the  floors  should  be  arranged 
to-  catch  and  carry  any  such  solution  to  a  sump  tank.  The 
loss  from  this  source  is  high  in  some  mills. 

The  apron  plates  may  be  of  raw  copper,  but  those  plated 
with  silver  are  much  preferred.  The  amount  of  silver  used 
per  square  foot  varies  from  1  to  3  oz.  One  ounce  is  suffi- 
cient for  the  upper  plates,  but  the  last  one  should  have 


102  STAMP  MILLING  AND  AMALGAMATION 

a  heavier  coating  as  the  scouring  action  over  it  is  much 
greater  than  above  where  more  amalgam  is  deposited. 
These  plates  are  from  52  to  56  in.  wide — slightly  wider 
than  the  discharge  to  them — and  can  be  obtained  in 
lengths  of  12  ft.,  but  are  commonly  used  in  sections  of  4 
ft.,  and  such  a  section  is  usually  spoken  of  as  a  plate.  The 
usual  thickness  is  %  in.,  though  plates  Vie  in-  thick  are 
in  use,  but  such  plates  are  easily  dented  by  objects  fall- 
ing on  them.  The  best  Lake  Superior  or  electrolytic  cop- 
per is  used  to  insure  purity  and  softness.  As  these  plates 
are  rolled  out,  which  makes  the  surface  dense,  in  pur- 
chasing raw  copper  plates  it  should  be  stipulated  that 
they  be  annealed;  annealing  softens  the  surface  of  the 
copper  that  the  mercury  may  be  better  absorbed.  They 
can  be  softened  by  heating  over  a  fire,  but  this,  if  not 
evenly  done,  is  liable  to  buckle  the  plate,  so  that  it  is  bet- 
ter not  to  try  it,  but  to  use  the  plate  as  it  is. 

Silver-plated  apron-plates  are  used  in  most  mills,  and 
are  preferred  by  amalgamators  as  being  easier  to  care  for. 
Experiments  have  shown  that  a  greater  saving  can  be 
made  with  silvered  plates  than  with  the  raw  copper.  Still, 
the  amalgamator  who  has  handled  both  and  who  knows 
how  to  care  for  the  raw  copper  plate,  believes  that  he  can 
amalgamate  as  well  with  one  as  with  the  other.  It  ap- 
pears that  the  personal  equation  of  the  amalgamator's 
ability  becomes  a  greater  factor  with  raw  copper  than 
with  silvered  plates. 

One  objection  to  the  use  of  raw  copper  plates  is  the 
trouble  necessary  to  get  them  in  suitable  condition.  It 
requires  a  few  weeks  before  they  become  saturated  with 
mercury  and  while  there  is  not  a  large  amount  of  gold 
carried  into  the  copper,  there  is  quite  an  amount  on  the 


SILVER-PLATED  COPPER  PLATES  103 

surface  of  the  plate  which  it  is  not  desirable  to  remove 
as  being  prejudicial  to  the  further  good  working  of  the 
plate.  The  silvered  plate  has  a  suitable  surface  already 
prepared.  The  mercury  sinks  slowly  through  this  silvered 
surface,  so  that  the  plate  on  the  start  does  not  have  to  be 
dressed  with  additional  mercury  as  often  as  the  raw  cop- 
per, but  eventually  it  absorbs  as  much  mercury  as  the 
raw  copper  plate.  The  amount  of  gold  carried  into  the 
copper  has  been  found  to  be  small,  assays  of  plates  in  long 
use  showing  about  one-sixth  ounce  of  fine  gold  per  square 
foot,  consequently  the  great  amount  of  bullion  coming 
from  old  plates  is  from  the  hard  scale  of  amalgam  on  the 
surface,  and  the  amalgamator  should  aim  to  keep  this  as 
small  as  possible.  Once  a  raw  copper  plate  is  well  started 
and  in  the  hands  of  a  good  amalgamator  it  should  do 
first-class  work,  but  the  bullion  returned  for  the  first 
month  in  operation  will  be  less  than  if  a  silvered  plate 
were  used.  The  difference  is  not  lost — the  gold  is  held 
on  the  surface  of  the  plate. 

To  prepare  a  raw  copper  plate  for  amalgamating,  it  is 
necessary  to  clean  the  surface  of  all  impurities  and  copper 
compounds,  and  to  a  lesser  extent,  to  soften  the  copper, 
after  which  it  is  amalgamated  by  rubbing  mercury  in. 
The  cleaning  is  done  by  thorough  scrubbing  with  fine 
sand,  wood  ashes,  or  slime,  using  wood  blocks,  rags,  or 
whisk  brooms.  Immediately  following  the  burnishing, 
weak  solutions  of  potassium  cyanide,  sal-ammoniac,  nitric 
acid,  or  caustic  soda  or  potash  is  used  with  a  view  to  soft- 
ening the  copper.  The  plate  can  be  amalgamated  without 
any  softening  in  this  way,  but  the  mercury  is  not  so  readily 
absorbed  and  consequently  it  is  a  longer  process.  For  this 
reason  one  of  these  chemicals  is  used  in  the  initial  process 


104  STAMP  MILLING  AND  AMALGAMATION 

of  amalgamating  of  a  new  raw  copper  plate,  even  by,those 
who  are  prejudiced  against  the  use  of  chemicals  on  the 
plates  under  other  conditions.  After  burnishing,  the  plate 
is  scrubbed  with  a  5%  solution  of  nitric  acid  followed  by 
another  scrubbing  with  a  2~y%%  solution  of  potassium  or 
sodium  cyanide — the  acid  being  washed  away  thoroughly 
that  it  may  not  re-act  to  neutralize  the  cyanide ;  the  cya- 
nide is  washed  away  and  followed  by  spraying  mercury 
(to  which  it  is  well  to  add  a  little  sodium-amalgam),  over 
the  plates  and  rubbing  for  a  long  time ;  mercury  is  added 
from  time  to  time  and  the  rubbing  continued  until  the 
plate  will  hold  no  more.  With  the  passing  of  time  mer- 
cury is  absorbed  into  the  plate  as  the  process  is  continued 
until  the  plate  is  saturated  with  mercury,  which  will  take 
about  two  weeks,  when  they  are  ready  for  the  dropping 
of  the  stamps.  After  the  first  amalgamating  no  acid 
should  be  used  on  the  plates  though  the  cyanide  solution 
may  be  used  if  the  mercury  does  not-  readily  amalgamate 
the  copper.  Following  the  first  amalgamating  or  just  be- 
fore dropping  the  stamps,  the  plate  should  be  coated  with 
silver  amalgam  or  the  ordinary  amalgam,  if  either  can  be 
obtained;  this  will  cause  the  plate  to  get  into  condition 
quickly  and  to  become  a  good  catcher  of  gold  almost  from 
the  start. 

Silver-plated  copper  plates  require  but  little  prepara- 
tion. They  should  be  washed  with  a  weak  solution  of  lye 
to  remove  any  grease  on  their  surface,  and  they  may 
be  polished  with  a  little  slime  or  ashes,  though  the  latter 
is  not  necessary.  A  long  and  thorough  rubbing  in  of  mer- 
cury is  desirable.  Silver  amalgam  or  the  ordinary  amal- 
gam should  be  applied,  if  obtainable.  If  amalgam  is  not 
applied,  then  some  sodium-amalgam  should  be  added  in 


ABSORPTION  BY  PLATES  105 

the  mercury  to  make  the  plate  more  active  at  the  start. 
When  starting  new  plates,  a  low-grade  ore  should  be  run 
through  first,  preferably  one  having  a  coarse,  easily  amal- 
gamated gold,  certainly  never  a  high-grade  ore  as  the 
catching  power  of  a  new  plate  is  low.  After  a  bed  of 
amalgam  is  started,  a  better  grade  of  ore  can  be  milled. 
A  much  discussed  question  is  how  to  prevent  the  lock- 
ing up  of  a  large  amount  of  bullion  in  the  plates.  To  melt 
the  plates  into  a  bar  requires  a  new  set  which  is  costly. 
While  the  old  set  will  more  than  pay  for  the  new,  all 
would  prefer  some  way  by  which  they  could  'eat  their 
cake  and  still  have  it. '  Of  this  locked-up  gold,  relatively 
only  a  small  part  is  absorbed  in  the  plates,  probably  1/6 
or  %  oz.  of  fine  gold  per  square  foot,  and  most  of  it  on 
the  surface  copper.  The  major  portion  of  the  gold  is  in 
the  form  of  a  hard  scale  of  amalgam ;  to  remove  this  scale 
the  copper  must  be  laid  bare  and  will  thus  have  its  amal- 
gamating tendency  reduced  for  some  time,  which  means 
a  loss.  To  resilver  the  plate  is  expensive.  The  wet  amal- 
gamator attempts  to  keep  the  thickness  of  this  scale  at  a 
minimum  by  softening  the  amalgam  down  to  the  silver- 
plating,  the  dry  amalgamator  accomplishes  it  by  chisel- 
ing the  amalgam  off  at  intervals ;  but  the  scale  is  certain 
to  accumulate  unless  they  are  willing  to  ruin  the  silver- 
plating  and  thereby  lose  its  advantages.  Certain  mills 
use  raw  copper  plates,  with  the  exception  of  the  last  plate 
which  is  silvered  on  account  of  the  scouring  action  of  the 
pulp.  These  copper  plates  are  periodically  sweated  and 
in  this  way  there  is  little  hard  scale  left  on  the  plates. 
Where  a  part  of  the  amalgam  is  spread  back  on  the  plates, 
the  practice  is  a  good  one,  but  where  the  stamps  are 
started  up  with  the  plates  bare,  it  must  be  condemned. 


106  STAMP  MILLING  AND  AMALGAMATION 

/ 

Where  it  is  considered  necessary  to  use  silvered  plates, 
and  it  is  still  desired  to  secure  this  bullion,  it  is  more  econ- 
omical to  scale  the  plates  and  have  them  resilvered  than 
to  melt  them  into  a  bar  for  shipment  to  the  bullion  buyers. 
There  is  no  really  good  method  for  removing  this  scale. 
Besides  sweating  and  chiseling,  the  plate  can  be  given 
repeated  scourings  with  pumice  stone  and  mercury,  and 
it  is  surprising  the  amount  of  gold  that  can  be  taken  from 
a  plate  in  this  way.  'Burning'  the  plate  is  resorted  to, 
which  consists  of  driving  off  the  mercury  by  heating  the 
under  side  of  the  plate,  after  which  the  gold  can  be  scaled 
off.  To  induce  the  scale  to  more  readily  come  off,  the 
plate  being  burnt,  may  be  subjected  to  a  scouring  with, 
or  a  bath  in,  some  chemical  that  will  form  a  compound 
with  copper,  thereby  softening  the  surface  copper.  Burn- 
ing a  plate  causes  it  to  buckle  and  puts  it  in  such  bad 
condition  that  plates  to  be  resilvered  or  re-used  should 
not  be  treated  in  that  way  unless  the  heat  applied  is  mod- 
erate or  there  are  facilities  for  restoring  the  plane  sur- 
face to  the  plate  again. 

Many  experiments  have  been  made  with  varying  tem- 
perature of  battery  water,  and  the  best  results  have  been 
secured  when  it  is  at  a  temperature  between  45  and  70° 
F.  Below  that  temperature  the  amalgam  tends  to  be- 
come hard  and  crystalline,  and  poor  amalgamation  may 
result,  though  the  heating  of  the  feed  water  is  seldom  at- 
tempted. Above  70°  the  amalgamation  of  the  gold  by 
the  plates  appears  to  be  promoted,  but  the  amalgam  be- 
comes so  liquid  that  it  is  hard  to  retain  it  on  the  plates, 
much  of  it  running  down  into  the  traps.  Amalgamating 
in  water  of  either  an  unusually  high  or  low  temperature 
bears  a  certain  analogy  to  amalgamating  wet  and  dry.  A 


TIME  TO  DRESS  PLATES  107 

high  temperature  causes  any  acidity  of  the  battery  water 
or  the  ore  to  act  more  promptly  in  staining  the  plates  by 
the  formation  of  various  compounds  with  the  copper.  The 
amalgamator  usually  manages  to  do  good  work  with  water 
of  either  a  high  or  low  temperature  by  keeping  the  amal- 
gam at  the  proper  consistence,  but  where  the  water  has 
a  considerable  variation  of  temperature  during  the  twen- 
ty-four hours,  it  is  impossible  to  vary  the  practice  accord- 
ingly. 

How  often  shall  the  plates  be  dressed?  Just  as  often 
as  needed  to  keep  them  in  good  condition ;  where  the  con- 
ditions are  not  greatly  variable  this  will  be  reduced  to 
dressing  at  stated  periods,  generally  12  hours  apart. 
Where  the  ore  is  low  grade,  'plating'  $1.50  to  $3  per  ton, 
the  plates  are  dressed  once  daily  in  the  morning,  though 
the  night  shift  may  soften  and  dress  the  upper  plate  of 
each  apron  if  it  becomes  hard.  This  refers  to  a  large 
mill,  in  a  small  one  two  dressings  will  be  made,  even  if  the 
plates  are  in  fair  condition,  on  the  principle  of  giving  the 
night  amalgamator  something  to  do.  With  ore  amalga- 
mating $4  to  $8,  two  or  three  dressings  will  usually  be 
made  during  the  twenty-four  hours,  while  with  rich  ore 
they  occur  a  few  hours  apart.  The  clean-up  of  the  amal- 
gam is  made  when  the  plates  are  dressed  in  the  morning. 
At  the  time  of  other  dressings  only  the  loose  crumbs  of 
amalgam  are  removed  unless  the  ore  is  rich.  The  plates 
should  be  rubbed  sufficiently  to  secure  an  even  texture 
of  the  amalgam,  in  theory  stopping  short  of  the  silver- 
plating,  while  allowing  as  little  of  the  hard  scale  to  form 
as  possible.  The  amalgam  should  be  worked  into  its  most 
active  condition  with  a  view  to  catching  gold,  and  further 
rubbing  is  superfluous.  Two  men  will  dress  a  plate  16 


108  STAMP  MILLING  AND  AMALGAMATION 

/ 

ft.  long  in  from  5  to  12  minutes,  depending  on  the  care  and 
refinement  used.  The  grade  of  ore  is  not  the  only  factor 
determining  the  number  of  dressings.  The  appearance 
of  stains,  or  the  coating  of  the  plate  by  galena  or  other 
sulphides,  or  by  the  semi-amalgamation  of  tellurium  or 
some  base  metal  will  at  once  call  for  a  dressing  to  remove 
the  fouling  substance.  Plates  that  have  been  treated  with 
strong  cyanide  solution  do  not  hold  mercury  well,  and 
some  time  after  the  dressing  the  mercury  may  collect  in 
drops,  or  tears,  that  work  down  the  plate,  when  the  plate 
is  said  to  'run,'  and  should  be  dressed  at  once. 

Some  amalgamators  are  troubled  by  the  amalgam  on 
the  apron-plates  becoming  abnormally  hard.  The  exact 
reason  for  this  cannot  be  given,  though  it  is  often  due  to 
certain  substances  in  the  ore,  but  usually  it  is  the  com- 
bination of  a  bare  plate,  dry  amalgamation,  and  the  too 
frequent  use  of  cyanide  in  dressing  the  plates.  The  amal- 
gamator, in  his  effort  to  prevent  this  hardening  of  the 
plate,  may  dress  it  wet  until  it  resembles  a  mirror,  but 
in  a  short  time  the  mercury  begins  to  form  in  visible 
globules  and  drain  off,  leaving  the  plate  hard.  To  correct 
this  trouble  all  chemicals  should  be  dispensed  with,  and 
the  principles  of  wet  amalgamation  practised  by  keeping 
a  thick  layer  of  amalgam  on  the  plates  and  dressing 
them  often,  giving  the  amalgam  a  long  and  hard  rubbing. 
This  amalgam  should  not  be  kept  too  wet  or  it  will  in- 
crease the  tendency  for  the  plate  to  run.  Unless  the 
trouble  lies  in  the  ore  or  water,  the  plate  will  regain  its 
normal  condition  under  this  treatment. 

The  placing  of  the  plates  at  a  distance  from  the  mortars 
instead  of  immediately  following  them  has  not  been  gen- 
erally successful ;  the  first  reason  being  that  it  is  hard 


DISTRIBUTION  OF  PULP  ON  PLATES  109 

to  distribute  the  pulp  evenly  across  the  width  of  the 
plates.  The  gold  appears  to  become  coated  with  slime 
in  the  short  time  that  elapses  between  its  leaving  the 
mortar  and  reaching  the  plates,  so  that  it  is  rendered  less 
readily  amalgamable.  During  its  passage  the  pulp,  to 
some  extent,  loses  its  homogeneity,  just  as  in  a  tailing 
flume  the  coarse  sand  settles  to  the  bottom.  When  the 
pulp  in  this  condition  reaches  the  plates,  the  coarser 
sand  tends  to  segregate,  while  the  finer  and  more  dilute 
portion  of  the  pulp  passes  on  and  over  the  plate ;  thus  a 
steeper  grade  of  plates  and  more  water  are  required.  The 
pulp  flows  down  over  the  plate  in  a  sheet,  or  flood,  rather 
than  with  the  rolling  over  and  over  wave  motion,  so  de- 
sirable, and  amalgamation  must  necessarily  be  poor,  espe- 
cially with  a  gold  difficult  to  catch.  The  pulp  as  it  leaves 
the  mortar  is  homogeneous,  though  the  tendency  of  the 
coarse  and  fine  particles  to  separate  is  noticeable  toward 
the  foot  of  a  long  plate. 

Where  the  ore  has  been  dry-crushed  and  subsequently 
watered  and  run  over  plates,  the  results  have  been  unsatis- 
factory in  many  cases,  partly  for  the  above  reasons  and 
partly  because  the  gold  has  become  fouled,  retarding 
amalgamation  by  being  coated  with  dirt,  or  air  bubbles, 
in  the  process  of  being  crushed. 

In  placing  plates  at  a  distance  from  the  mortars,  well 
considered  arrangements  should  be  made  for  securing  an 
even  distribution  of  pulp  to  and  across  the  width  of  them. 
Comparative  tests  should  be  conducted  against  a  plate 
set  in  front  of  the  mortars.  Sufficient  mill  space  should 
always  be  provided  to  allow  plates  to  be  placed  directly 
in  front  of  the  mortars,  if  thought  desirable.  With  a 
clean  ore  that  slimes  but  little  and  coarse  gold,  it  should 


110  STAMP  MILLING  AND  AMALGAMATION 

be  possible  to  amalgamate  satisfactorily  at  a  distance 
from  the  mortar,  but  with  a  sliming  ore  and  fine  gold  it 
is  doubtful.  Possibly  the  practice  at  the  Homestake  'mills 
at  Lead,  South  Dakota,  is  an  exception,  as  there  has  been 
in  use  an  auxiliary  ' plate  house'  of  large  dimensions  en- 
tirely separated  from  the  mills,  in  which  successful  amal- 
gamation is  carried  on  with  an  ore  notorious  for  its 
sliming  proclivities.  However,  the  usual  plates  are  in 
use  in  front  of  the  mortars  in  the  Homestake  mills.  In 
this  connection  the  question  of  what  constitutes  satis- 
factory amalgamation  presents  itself,  and  it  may  be  re- 
marked that  there  is  a  difference  in  millmen.  One  man 
will  be  satisfied  with  the  percentage  of  gold  he  is  saving 
and  claim  that  no  more  can  be  saved,  while  another  man, 
on  taking  the  same  mill  and  recovering  the  same  per- 
centage of  the  gold,  may  be  dissatisfied  with  results  and 
consider  that  more  gold  can  be  saved  by  greater  refine- 
ment of  methods.  In  cyanide  practice  it  is  possible  in  a 
majority  of  cases  to  determine  by  laboratory  tests  the 
extent  to  which  the  gold  can  be  dissolved,  and  the  metal- 
lurgist can  be  required  to  closely  approximate  this  in  the 
actual  work.  But  in  amalgamation  by  small  tests  it  is 
impossible  to  determine  even  approximately  the  maximum 
amount  of  the  gold  that  can  be  amalgamated,  and  the  re- 
sults that  can  be  attained  in  actual  mill  practice  are  the 
only  real  index  as  to  the  ability  of  the  millman  and  the 
thoroughness  with  which  the  amalgamation  is  being  ac- 
complished. 

For  carrying  the  tools  used  in  dressing  the  plates  and 
for  holding  the  amalgam  recovered,  a  deep  iron  kettle  is 
used,  or  sometimes  a  gold-pan.  A  wedgewood  or  glazed 
sheet-iron  bowl  for  cleaning  the  amalgam  is  also  required. 


FEEDING  QUICKSILVER  111 

For  spraying  the  mercury  on  the  plates,  a  strong  bottle 
with  a  piece  of  canvas  or  a  double  thickness  of  muslin 
tied  over  the  mouth  is  used.  Suitable  bottles  may  be 
made  of  iron  by  screwing  a  cap  over  one  end  of  a  1%  °r 
2-in.  iron  pipe.  Mercury  is  also  occasionally  tied  up  in 
a  canvas  bag  in  quantity  about  the  size  of  an  egg ;  such  a 
bag  is  kept  in  a  cup  to  prevent  loss  of  the  mercury.  For 
feeding  the  mercury  into  the  mortar,  an  amalgamator's 
spoon  is  used  to  measure  it  from  a  cup;  this  spoon  is  of 
wood,  much  like  a  mustard  spoon;  the  bowl  is  bored  or 
burned  in  it  to  hold  mercury  to  the  size  of  a  small  pea. 
The  amalgamator  throws  the  quick  into  the  mortar 
through  the  feed  slot  at  the  back,  a  half  or  full  spoon, 
or  more,  according  to  the  amount  he  considers  necessary 
from  the  appearance  of  the  plates.  Some  head  amalgam- 
ators make  use  of  a  chart  having  a  blank  space  for  each 
battery,  and  write  on  these  blanks  the  amount  of  mercury 
to  be  fed  to  each  mortar  by  their  assistants  on  the  differ- 
ent shifts.  This  is  an  unsatisfactory  method  and  seldom 
found,  for  even  an  inexperienced  man  can  learn  to  feed 
mercury  properly  under  instruction  in  a  short  time. 

As  few  chemicals  as  possible  should  be  used  in  the 
treatment  of  plates.  Their  continued  employment  is 
analogous  to  the  use  of  stimulants  by  a  man.  Apparently 
beneficial,  their  after  effects  usually  more  than  counteract 
their  fancied  or  real  temporary  advantages.  The  mercury 
should  be  'pickled'  in  weak  nitric  acid  to  remove  impuri- 
ties, as  has  been  explained  before,  but  all  traces  of  the 
acid  should  be  washed  out  before  applying  the  mercury 
to  the  plates.  Greasy  plates  should  be  scrubbed  with  a 
weak  solution  of  lye,  which  will  neutralize  or  remove  it. 

Potassium  cyanide  has  been  used  extensively  in  the  past, 


112  STAMP  MILLING  AND  AMALGAMATION 

but  its  use  must  be  condemned  as  wrong  in  theory  and 
harmful  in  results.  Cyanide  of  potassium  solution'  neu- 
tralizes grease,  in  which  it  is  beneficial;  it  also  dissolves 
the  compounds  of  copper,  allowing  them  to  be  washed 
away,  in  which  it  is  also  beneficial;  but  it  goes  farther; 
it  dissolves  the  metallic  copper  itself  and  thereby  pits 
the  surface  of  the  plate.  This  pitting  of  the  plate  and  the 
formation  of  compounds  of  copper  and  cyanide  that  are 
only  partly  removed  by  the  water,  increases  the  tendency 
of  the  plate  to  readily  absorb  mercury  (is  said  to  soften 
the  plate),  but  these  beneficent  results  are  only  temporary, 
as  the  plate  becomes  still  harder  and  the  mercury  tends 
to  ooze  out  of  the  amalgamated  plate,  as  may  be  observed 
in  plates  that  have  been  continually  treated  with  strong 
cyanide  solution,  while  the  copper  compounds  that  have 
been  formed,  oxidize  to  tarnish  the  plate  at  every  oppor- 
tunity. When  the  solution  is  used,  it  should  be  weak, 
that  only  the  copper  compounds  may  be  attacked,  and 
that  the  raw  copper  may  be  unacted  upon,  and  it  should 
be  thoroughly  washed  away  immediately  after  using,  that 
none  of  the  copper  compounds  or  salts  may  remain.  A 
2l/2%  solution — less  than  one-half  ounce  potassium  cya- 
nide to  a  pint  of  water — is  the  strength  generally  used, 
and  a  still  weaker  solution  is  preferable.  Plates  that  have 
been  long  treated  with  strong  cyanide  solution  are  diffi- 
cult to  handle,  especially  if  little  amalgam  is  kept  on 
them,  and  that  in  a  hard  condition.  In  general,  any  acid 
or  chemical  that  will  form  a  compound  with  copper  should 
not  be  applied  to  the  plates,  while  the  copper  should  be 
kept  well  covered  with  amalgam  that  tarnishing  due  to 
oxidation  by  the  air  or  indirectly  by  the  acidity  of  the 
water  or  ore  may  be  minimized.  Where  the  pulp  is  acid 


STAINED  PLATES  113 

from  the  water  or  the  ore,  it  is  claimed  that  iron  strips 
down  the  side  of  the  plate  will  cause  the  acidity  to  affect 
the  iron  instead  of  the  copper,  keeping  the  plates  free 
from  stains  in  addition  to  exerting  a  helpful  galvanic 
effect,  but  it  is  doubtful  if  it  would  be  of  material  ad- 
vantage, as  only  a  small  part  of  the  pulp  comes  in  direct 
contact  with  the  iron. 

The  stains,  the  so-called  'verdigris/  which  appear  on 
the  plates  and  are  a  source  of  grief  to  many  amalgama- 
tors, are  in  nearly  every  case  an  oxide  of  copper,  though 
they  may  be  carbonate  in  some  instances.  These  stains  are 
due  to  copper  in  the  amalgam  which  is  turned  into  an  oxi- 
dized compound  by  the  air  or  water.  This  copper  may  be 
amalgamated  together  with  the  gold  from  the  ore,  or  its 
salts  may  have  contaminated  the  mercury,  but  ordinarily 
the  stains  rise  from  the  plate  due  to  too  thin  a  film  of  amal- 
gam, to  the  use  of  chemicals,  or  to  the  pulp  containing 
substances  that  have  formed  copper  compounds  which 
have  later  oxidized.  An  important  source  of  this  copper 
is  the  shells  of  detonators  used  in  blasting.  The  remedy 
is  to  use  purified  mercury,  dressing  the  plates  as  often  as 
the  stains  appear  until  a  good  film  of  amalgam  is  accu- 
mulated over  the  plate,  and  especially  over  the  spots  which 
most  frequently  tarnish.  Solutions  of  potassium  cyanide, 
sal-ammoniac,  and  of  the  acids  are  used,  especially  the 
first,  to  dissolve  these  stains  that  they  may  be  washed 
away,  but  as  it  is  impossible  to  prevent  the  chemicals  from 
continuing  to  act  further,  they  should  not  be  employed. 
The  use  of  silvered  instead  of  raw  copper  plates  will  or- 
dinarily obviate  this  trouble.  However,  it  largely  depends 
upon  the  ability  of  the  amalgamator. 

Bare  spots  on  the  plates  are  due  to  the  amalgam  hav- 


114  STAMP  MILLING  AND  AMALGAMATION 

ing  been  removed  too  close  to  the  copper  by  chiseling,  or 
they  may  be  started  by  a  tarnishing  spot.  They  also'  may 
appear  on  the  lower  apron  plate  due  to  the  scouring  of  the 
pulp  as  has  been  mentioned  before.  These  spots  require 
careful  treatment  for  a  little  while.  The  copper  should  be 
burnished  with  fine  grit,  such  as  wood  ashes  or  slime,  un- 
til the  pure  copper  is  exposed,  when  the  adjacent  amalgam 
should  be  worked  over  it  and  rubbed  in  well.  The  depo- 
sition of  the  amalgam  should  be  coaxed  from  the  edges 
to  the  center.  A  little  sodium  can  be  used  to  advantage 
in  the  mercury  and  this  compound,  sodium-amalgam,  ap- 
plied to  these  spots,  as  it  will  promote  the  attachment  be- 
tween the  amalgam  and  the  plate  and  aid  in  the  recovery 
of  additional  amalgam.  Cyanide  and  acid  solutions  should 
not  be  used  on  these  spots. 

Amalgamators  have  various  'dopes'  and  nostrums,  se- 
cret and  otherwise,  for  applying  to  the  mercury  and  plates 
the  action  of  which  they  themselves  but  little  understand, 
especially  in  regard  to  the  chemical  reactions  from  which 
they  must  derive  their  virtue,  if  they  possess  any.  It  is 
best  to  dispense  with  them  all.  There  is  only  one  *  dope  *  or 
panacea  for  the  ills  of  amalgamation,  and  that  is  a  thick 
bed  of  amalgam,  kept  in  an  active  condition  and  free  from 
foreign  substances.  This,  together  with  a  vigorous  appli- 
cation of  'elbow  grease,'  produces  the  best  .results. 

The  mercury  is  sometimes  'loaded'  by  adding  sodium 
amalgam,  up  to  the  point  where  the  mercury  just  com- 
mences to  amalgamate  a  bright  nail.  Sodium  amalgam  is 
prepared  by  heating  the  mercury  in  a  glazed  dish,  or  better 
still,  a  quicksilver  flask,  the  top  of  which  has  been  cut 
off,  forming  a  deep  pot.  The  making  of  sodium  amalgam 
is  attended  with  danger  and  should  be  conducted  carefully. 


MAKING  SODIUM  AMALGAM  115 

Heat  the  mercury  to  about  300 °F.  and  cut  small  chips,  the 
size  of  a  good-sized  pea  from  the  stick  of  sodium,  han- 
dling with  a  pair  of  tongs.  Drop  but  one  chip  into  the 
heated  mercury  at  a  time.  A  slight  explosion  should  fol- 
low. If  it  does  not,  stir  it  gently  with  a  wooden  paddle, 
which  will  hasten  the  flash.  Then  add  another  chip  in  like 
manner,  up  to  3%  of  the  weight  of  the  mercury  to  be  thus 
treated.  This  will  crystallize,  forming  a  solid  amalgam 
when  cold.  Keep  the  face  away  from  the  flask,  both  on 
account  of  the  flashing  sodium  and  to  avoid  the  mercurial 
vapor  that  is  likely  to  arise  from  the  heated  pot.  This 
amalgam  should  be  kept  in  air-tight  bottles  that  it  may 
not  decompose,  and  should  be  added  in  small  quantity  to 
the  mercury  as  required.  The  effect  of  the  sodium  in  mak- 
ing the  mercury  so  active  in  amalgamating  is  not  fully 
understood,  but  is  supposed  to  be  largely  due  to  its  re- 
ducing action.  It  reduces  the  oxides  and  other  compounds 
of  the  base  metals,  causing  them  to  amalgamate  with  the 
mercury.  It  is  but  little  used  by  practical  amalgamators 
as  it  causes  the  amalgam  to  'freeze'  to  the  iron  and  steel 
surfaces  of  the  mortar  through  amalgamating  with  them, 
while  so  much  fine  iron  and  steel  and  sulphide  are  caught 
with  the  plate  amalgam,  that  these  surfaces  become  foul. 
It  is  productive  of  an  increased  quantity,  but  a  lower  grade 
of  amalgam.  Besides  being  useful  in  covering  bare  spots, 
it  is  of  benefit  in  starting  new  plates — making  them  more 
active. 

Silver  amalgam  can  be  prepared  by  dissolving  silver 
coin  or  other  silver  in  dilute  nitric  acid.  To  this  solu- 
tion add  the  mercury  and  a  few  bright  nails,  keeping 
the  acid  weak.  The  silver  will  be  deposited  on  the  mer- 
cury forming  an  amalgam.  Unless  the  amount  of  copper 


116  STAMP  MILLING  AND  AMALGAMATION 

in  the  silver  is  large,  its  presence  is  not  material.  This 
copper  can  be  removed  by  precipitating  the  silver  from 
the  nitrate  solution  by  adding  a  solution  of  common  salt 
up  to  the  point  where  no  more  precipitation  occurs,  then 
washing  the  precipitate  until  all  green  color  of  copper 
is  gone,  after  which  very  dilute  nitric  or  sulphuric  acid, 
mercury,  and  bright  nails  are  added.  The  finely  divided 
precipitated  silver  may  be  separated  by  filtering  and 
worked  up  with  the  mercury  later.  A  rough  method  of 
making  silver  amalgam  is  to  reduce  the  silver  to  filings 
and  amalgamate  it  by  mixing  with  mercury. 

After  the  amalgam  is  removed  at  the  daily  cleaning  of 
the  plates,  it  is  necessary  to  clean  it  by  removing  the  sand, 
iron,  sulphide,  and  base  metal  dross.  This  is  done  by 
grinding  it  in  a  wedgewood  or  iron  bowl  with  enough 
mercury  to  make  it  quite  liquid.  The  impurities  rise  to  the 
top  where  they  are  carried  over  the  side  of  the  bowl  into  a 
gold  pan  in  which  the  bowl  sets,  by  a  stream  of  water  from 
a  hose,  or  they  are  taken  off  by  a  coarse  sponge.  After  the 
quartz  and  dirt  and  dross — impure  and  foul  amalgam — has 
been  taken  off,  a  magnet  is  passed  through  the  liquid  amal- 
gam several  times  to  remove  the  fine  iron  and  steel.  It  is 
now  poured  into  a  wet  canvas  or  a  double  thickness  of 
stout  drilling  lining  a  bowl.  The  cloth  is  gathered  up  by 
the  corners  and  twisted  tight  forcing  the  mercury  through 
the  fabric,  leaving  the  amalgam  behind  in  the  cloth.  This 
process  of  squeezing  or  wringing  the  amalgam  is  contin- 
ued, the  globules  of  mercury  being  washed  off  by  aid  of 
water  into  the  bowl  beneath.  When  all  the  mercury  has 
been  expressed  that  the  operator  can  wring  from  it,  the 
cloth  is  spread  out  exposing  a  ball  of  hard  amalgam 
within.  The  ball  is  rolled  around  in  the  cloth  to  pick  up 


CLEANING  AMALGAM  117 

the  loose  flakes  of  amalgam;  after  which  the  ball  is 
weighed,  wrapped  in  paper  on  which  is  usually  marked 
the  date  and  weight,  and  locked  up  until  the  monthly  or 
semi-monthly  retorting  and  melting.  The  mercury  pass- 
ing through  the  cloth  will  carry  some  gold,  but  this  is  not 
considered  undesirable  as  such  mercury  is  more  active  in 
promoting  amalgamation  than  that  which  is  gold-free. 
The  amount  of  gold  so  retained  can  be  reduced  by  sqeez- 
ing  the  amalgam  through  a  less  porous  material,  such  as 
chamois. 

Some  amalgamators  clean  the  amalgam  by  adding  suf- 
ficient mercury  to  make  it  soft  and  mushy,  when  it  is 
dumped  upon  the  upper  apron  plate,  where  it  is  puddled 
by  the  fingers,  or  a  rubber,  until  it  sticks  to  the  plate  in 
one  mass.  Water  is  turned  on  from  a  hose  which,  in  con- 
nection with  the  puddling  washes  the  amalgam  clean.  Af- 
ter using  the  magnet,  the  amalgam  is  scooped  up  and 
transferred  to  the  straining  cloth.  This  is  a  poor  method 
though  the  amount  of  amalgam  lost  is  small  when  the 
treasure  box  is  used ;  it  causes  a  loss  of  running  time  and 
leaves  a  wet  spot  on  the  plate.  The  easiest  way  is  to  use 
a  gold  pan  haying  an  amalgamated  copper  bottom,  when 
removing  the  amalgam  from  the  plates.  Then,  when  at 
leisure,  work  this  amalgam  up  at  the  clean-up  sink  in  the 
same  manner  as  on  an  apron  plate.  Grinding  with  an  ex- 
cess of  mercury  in  a  wedgewood  bowl  gives  the  cleanest 
amalgam.  The  dross  or  impure  mercury  and  the  rich  sul- 
phide collecting  from  the  daily  cleaning  of  the  amalgam 
should  be  ground  with  mercury  when  a  quantity  has  col- 
lected. All  of  the  debris  and  refuse  from  these  cleanings 
should  finally  go  to  the  tank  of  the  clean-up  sink,  to  be 


118  STAMP  MILLING  AND  AMALGAMATION 

run  through  the  clean-up  barrel  later,  or  sent  through  the 
mortars  in  the  absence  of  a  clean-up  barrel. 

At  the  monthly  clean-up  the  apron-plates  are  cleaned 
before  the  mortars  are  opened.  Where  there  is  consider- 
able gold  in  the  mortar  sand,  this  sand  is  removed  once  a 
month  or  on  general  clean-up  day.  The  battery  is  stamped 
out  and  hung  up.  A  platform  of  boards  with  cleats  to  fit 
the  apron-table  is  placed  over  the  first  plate  to  work  upon 
without  marring  it.  The  screen  is  removed.  The  splash, 
lip,  and  inside  plates  are  removed  to  the  clean-up  room 
to  be  chiseled  and  scraped,  which  is  done  by  means  of 
old  files  forged  down  to  chisel  ends  and  ground  sharp  on 
a  grindstone.  A  putty  knife  is  a  useful  tool  for  this  pur- 
pose. The  pulp  lying  on  the  dies  is  shoveled  into  boxes 
or  tubs  to  be  returned  to  the  mortar,  it  usually  containing 
too  little  amalgam  to  be  treated.  The  sand  about  the  dies 
is  dug  out  with  bars  and  together  with  that  underneath 
the  dies  is  carried  in  pails  or  tubs  to  the  clean-up  room, 
or  to  the  clean-up  barrel  or  pan.  The  dies  are  pried  up 
and  removed  to  get  the  amalgam-bearing  sand  around  and 
beneath  them.  After  cleaning  out  all  the  sand  and  knock- 
ing off  any  amalgam  adhering  to  the  sides  of  the  mortar, 
or  to  the  shoes  and  bosses,  the  dies  which  have  been 
washed  and  examined  for  any  deposits  of  amalgam  are  re- 
turned to  the  mortar;  the -height  of  drop  adjusted,  chuck- 
block,  screen,  and  plates  put  in  position,  and  the  battery 
started  up.  It  is  aimed  to  put  the  new  shoes  and  dies  in 
at  this  time  whenever  possible,  and  the  mercury  traps  are 
also  then  cleaned.  At  some  mills  the  battery  sand  from 
all  the  mortars,  when  low  in  amalgam,  is  sent  through  one 
mortar  having  a  high  discharge,  and  only  the  sand  from 
this  mortar  is  taken  to  the  clean-up  room,  but  it  is  not  con- 


CLEANING-UP  119 

sidered  good  practice.  At  the  clean-up  room  this  sand  is 
either  panned  down  in  an  ordinary  gold  pan  or  is  run 
through  a  rocker  to  separate  and  collect  the  amalgam,  the 
sand  tailing  being  fed  through  the  mortar  before  the  next 
clean-up. 

Where  a  clean-up  barrel  is  used,  the  mortar  and  mer- 
cury-trap sands,  together  with  that  in  the  clean-up  sink 
from  the  daily  cleaning  of  the  amalgam,  are  put  in  the 
barrel  with  40  Ib.  or  more  of  mercury — sufficient  to  in- 
sure the  amalgam  being  liquid — also  sufficient  water  to 
make  a  sludge  or  thick  pulp.  To  this  is  added  a  little  lye 
to  'cut'  the  grease  and  keep  the  mercury  in  condition,  and 
many  pieces  of  iron  and  steel  to  act  as  grinders  or  mul- 
lers,  such  as  cannon  balls,  broken  stem  ends  and  shoe 
shanks.  Pieces  of  hard  quartz  or  other  rock  answer  well. 
The  barrel  is  now  revolved  from  3  to  8  hours,  depending 
on  the  ideas  of  the  amalgamator  and  time  available,  at  a 
speed  not  exceeding  15  revolutions  per  minute — the  higher 
the  speed,  the  greater  the  tendency  of  the  amalgam  to 
flour.  After  grinding,  the  barrel  is  slowed  down  in  order 
that  the  soft,  liquid  amalgam  may  collect;  it  is  finally 
stopped  with  the  manhole  uppermost  and  a  small  plugged 
opening  below.  The  manhole  is  opened  first  to  give  vent 
to  any  gases  that  may  have  formed,  after  which  the  plug 
below  is  removed  to  allow  the  amalgam  to  run  into  a  deep 
receptacle  set  underneath.  The  sand  is  slowly  sluiced  out 
of  the  barrel  with  sufficient  water  to  make  a  thin  pulp 
that  will  not  carry  away  any  amalgam.  It  is  run  through 
a  coarse  screen  to  remove  nails  and  other  small  fragments 
of  iron,  to  riffles  and  an  amalgamated  plate  for  catching 
any  escaped  amalgam,  and  is  finally  caught  in  a  box  or 
tank.  A  batea,  a  shallow  wooden  bowl,  is  superior  to  rif- 


120  STAMP  MILLING  AND  AMALGAMATION 

fles  or  a  plate  for  catching  any  amalgam  that  has  over- 
flowed from  the  pail.  The  batea  may  be  characterized  as 
a  mechanically-operated  gold-pan  that  retains  the  amal- 
gam and  pans  off  the  sand.  The  amalgam  recovered  is 
cleaned  of  iron  by  the  magnet  and  strained  into  balls 
through  canvas  in  the  usual  manner.  The  sand  is  even- 
tually carried  back  to  the  mortar  as  it  always  contains  a 
little  amalgam.  Grinding  pans  are  used  at  some  mills  in- 
stead of  barrels  for  cleaning  the  sand  and  amalgam;  the 
only  variation  in  the  general  treatment  being  that  necessi- 
tated by  the  difference  in  their  construction ;  both  are  dis- 
pensed with  at  many  mills  as  they  are  considered  by  some 
to  flour  the  amalgam,  while  on  the  other  hand  some  mills 
are  equipped  with  both.  The  amalgam  chiseled  from  the 
plates,  or  that  panned  out  of  the  sand,  is  ground  up  in  a 
large  wedgewood  bowl  or  in  an  iron  hand-mortar  with  an 
excess  of  mercury.  Warm  water  is  often  used  in  clean- 
ing the  amalgam  that  it  may  become  softer  and  liberate 
more  freely  the  impurities  mechanically  held  or  suspended 
in  it,  and  that  it  may  be  squeezed  drier.  Some  amalgama- 
tors, though  not  many,  add  the  amalgam  chiseled  from 
the  plates  at  the  clean-up  to  the  sand  in  the  barrel  and 
clean  it  in  that  way,  but  it  is  not  good  practice  to  thus 
take  any  chances  of  its  being  lost  or  stolen. 


CHAPTER  VIII 

The  amalgam  is  retorted  to  free  the  gold  from  the  mer- 
cury. This  is  accomplished  in  a  closed  cast-iron  vessel 
having  a  tube  leading  from  it  to  carry  away  the  volatilized 
mercury  and  deliver  it  to  a  kettle  re-condensed  in  liquid 
form.  These  vessels  are  called  retorts,  and  may  be  either 
large  cylinders  solidly  set  in  a  foundation  of  brick  over  a 
fire-box,  or  they  may  be  small  portable  affairs  that  are 
placed  over  a  fire  made  in  a  temporary  furnace  of  brick 
or  stone,  or  even  in  the  open  air  on  the  ground.  The  in- 
side of  the  retort  is  lined  with  three  or  four  thicknesses  of 
paper,  with  chalk,  or  with  wood  ashes,  to  prevent  the 
gold  from  sticking  to  the  sides.  In  the  retort  the  balls  of 
amalgam  are  placed  so  that  it  will  not  be  more  than  three- 
quarters  full  when  the  cover  is  placed  on.  If  the  '  sponge, ' 
as  the  metal  after  retorting  is  called,  is  to  be  shipped  with- 
out being  melted  into  a  bar,  the  amalgam  is  packed  down 
tight  to  make  a  solid  mass  of  it,  and  a  hole  is  forced  down 
through  the  center  to  enable  the  mercury  the  better  to  es- 
cape from  within  the  mass.  If  the  retorted  metal  is  to 
be  melted  into  a  brick  before  shipping,  the  balls  are  put 
in  loosely  since  that  will  allow  the  volatilized  mercury  to 
readily  escape  and  the  sponge  to  be  easily  broken  up  for 
convenience  in  handling  before  melting.  A  ring  of  lute 
made  of  fire-clay,  wood  ashes,  and  a  little  salt  is  placed 
between  the  cover  and  body  of  the  retort  to  insure  an  air- 
tight joint.  The  retort  is  set  in  a  furnace,  on  a  tripod,  or 
is  carefully  propped  up  in  the  open,  and  a  slow  wood  fire 
started  about  it.  This  fire  is  gradually  raised  until  the 


122  STAMP  MILLING  AND  AMALGAMATION 

retort  finally  becomes  cherry  red;  the  heat  being  applied 
all  about  the  retort,  not  on  the  bottom  alone.  The-  pipe 
coming  out  of  the  retort  passes  through  an  outer  pipe, 
and  in  the  space  between  them  cold  water  is  constantly 
running  which  condenses  the  volatilized  mercury  to  the  li- 
quid state.  When  using  small  retorts  gunnysacks  wrapped 
about  the  pipe,  to  which  cold  water  is  continually  applied, 
may  successfully  be  used.  A  vessel  filled  with  water  is 
placed  at  the  lower  end  of  the  pipe  to  catch  the  condensed 
mercury.  A  cloth  should  be  wrapped  about  the  end  of  the 
tube  forming  an  extension  of  it.  This  extension  should 
dip  beneath  the  surface  of  the  water  with  which  the  pail 
is  filled  to  overflow  as  the  mercury  condenses.  Care 
should  be  taken  that  the  lower  end  of  the  pipe  itself  does 
not  project  into  the  water  as  there  is  a  tendency  for  the 
atmospheric  pressure  to  force  the  water  up  into  the  pipe 
at  times,  due  to  the  diminution  of  the  volume  of  gas  in  the 
retort  caused  by  the  fire  being  checked  or  dying  down; 
in  such  case  the  cloth  will  be  drawn  against  the  pipe  and 
air  drawn  through  the  pores  of  the  cloth,  whereas,  if  the 
end  of  the  pipe  were  in  the  water,  the  water  might 
ascend  into  the  retort  and  an  explosion  result.  If  the  end 
of  the  pipe  were  exposed  without  the  cloth,  some  volatil- 
ized mercury  might  escape  if  the  condenser  was  not  work- 
ing properly,  endangering  salivation.  Retorting  will  take 
from  2l/2  to  5  hours.  The  fire  should  be  continued  for 
20  minutes  after  the  mercury  ceases  to  condense  in  the 
pipe  as  ascertained  by  tapping  the  pipe  and  watching  for 
the  mercury  to  drop  out.  When  distillation  is  complete 
the  fire  is  withdrawn  and  the  retort  allowed  to  cool.  There 
is  always  danger  of  salivation  if  the  retort  is  opened  while 
hot.  It  is  practically  impossible  to  drive  off  the  last  of 


RETORTING  MERCURY  123 

the  mercury  and  it  should  not  be  attempted  by  raising 
the  heat,  as  such  heat  may  result  in  a  partial  fusion  of  the 
gold  causing  it  to  stick  to  the  retort,  and  is  also  destruc- 
tive of  the  retort.  Retorting  with  an  assay  furnace  or  on 
a  blacksmith's  forge  invariably  results  in  using  too  high 
a  heat.  Where  the  amount  of  amalgam  to  be  retorted  is 
large  the  oval  or  cylindrical  retorts  of  large  capacity  above 
referred  to  are  used  in  specially  built  furnaces.  The  amal- 
gam is  placed  in  cast-iron  trays  or  separated  by  parti- 
tions of  plate  iron  in  these  retorts,  these  trays  or  plates 
being  well  chalked  or  painted  with  ashes  or  other  mixture 
and  well  dried  to  prevent  the  gold  from  sticking  to  them. 

If  the  amalgam  has  been  properly  cleaned  and  retorted, 
the  sponge  will  show  a  gold  color  and  require  a  minimum 
amount  of  flux  in  the  melting.  Blackness  indicates  that 
the  amalgam  was  poorly  cleaned.  A  pale  whitish  color 
shows  that  it  still  contains  mercury,  and  a  bluish  color 
generally  indicates  the  presence  of  lead,  usually  babbitt. 
Retorting  should  be  done  in  the  open,  or  with  the  win- 
dows of  the  retort  room  open,  to  lessen  the  danger  of  be- 
ing salivated,  though  there  is  little  danger  if  proper  pre- 
cautions are  taken. 

The  percentage  of  metal  that  is  obtained  from  the  amal- 
gam by  retorting  and  melting  it  into  a  bar  depends  upon 
the  amount  of  impurities  in  the  amalgam,  and  to  a  much 
greater  extent  on  the  size  of  the  particles  of  gold.  Coarse 
gold  does  not  require  as  much  mercury  to  amalgamate,  or 
cement  it,  as  a  fine-grained  gold.  About  30  to  40%  is  the 
usual  amount  of  bullion  obtained  from  the  amalgam  of 
ordinary  gold.  With  coarse  gold  as  high  as  65%  of  bul- 
lion has  been  obtained,  while  with  an  extremely  fine  gold 
the  amalgam  may  run  as  low  as  20%  of  bullion.  Harder 


124  STAMP  MILLING  AND  AMALGAMATION 

squeezing  in  the  canvas  does  not  materially  lessen  the 
excess  of  mercury.  The  use  of  sodium  amalgam  will  in- 
crease the  amount  of  amalgam  by  amalgamating  the  fine 
iron,  steel,  and  sulphide.  In  one  case,  with  a  fine  gold 
that  ordinarily  retorted  22%  bullion,  sodium  amalgam  was 
used  in  starting  new  plates,  resulting  in  an  increased  yield 
from  these  plates  that  assayed  only  12%  fine  bullion. 

Before  melting  the  retorted  metal,  the  black-lead  cru- 
cible must  be  annealed  by  driving  off  all  the  contained 
moisture,  or  the  sudden  heating  of  this  moisture  will  burst 
the  pot.  For  this  purpose  the  pot  is  kept  in  a  warm  place, 
such  as  under  or  over  a  stove  or  boiler  for  a  week  or  more, 
when  it  is  placed  directly  in  the  stove  or  in  the  boiler  fire 
for  some  time,  after  which  it  can  be  used  with  safety.  The 
pot  is  placed  in  the  furnace  fire  and  when  sufficiently 
heated  to  melt  the  metal,  the  flux  is  added.  After  the  flux 
has  become  molten,  the  retorted  metal  sponge  is  added  in 
pieces  as  fast  as  it  melts  down  until  the  whole  is  melted, 
employing  a  long  scoop  or  blower  in  handling  the  pieces 
of  gold  sponge.  The  quantity  of  the  flux  and  its  character 
will  depend  upon  the  cleanness  of  the  sponge  after  re- 
torting and  the  nature  of  the  impurities.  The  amount  of 
flux  used  and  the  proportions  vary  with  each  melter  and 
can  be  determined  only  in  an  empirical  way,  by  knowing 
the  theory  of  fluxing  and  then  guessing  at  the  amount  and 
character  of  the  impurities.  Should  the  amount  of  flux 
appear  too  small,  as  melting  proceeds,  more  can  be  added 
at  any  time,  while  an  excess  does  no  harm,  nitre  excepted. 
The  pot  should  be  provided  with  a  cover  which  is  kept  in 
place  during  melting,  except  when  removed  for  observa- 
tion or  for  stirring  the  melt. 

The  experienced  melter  on  a  clean  retorted  metal  will 


FLUXING  RETORTS  125 

use  little  or  no  flux,  while  the  novice  may  use,  on  a  some- 
what base  retort,  flux  to  the  amount  of  5  or  10%  of  the 
metal.  The  average  melter  employs  borax-glass  and  bi- 
carbonate of  soda  in  approximately  equal  proportions  by 
bulk.  The  professional  melter  confines  himself  largely  to 
borax-glass.  The  principal  impurities  to  be  fluxed  off  are 
oxide  of  iron,  sand,  and  a  little  sulphur.  Borax  dissolves 
the  metallic  oxides  forming  borates  of  the  bases;  soda 
acts  as  a  desulphurizer  and  forms  sodium  silicates  with 
the  sand;  together  they  slag  off  the  impurities  and  cause 
the  metal  to  melt  down  rapidly.  Theoretically  one  part 
of  base  requires  about  two  of  borax-glass,  and  one  part 
of  silica  (sand)  should  have  about  five  of  bicarbonate  of 
soda.  It  is  preferable  to  use  an  excess  of  borax-glass. 

For  taking  care  of  small  or  medium  amounts  of  iron, 
in  addition  to  the  use  of  borax,  silica  in  the  form  of  fine, 
granular  quartz  tailing,  may  be  used  to  form  an  iron  sili- 
cate. Theoretically  one  part  of  iron  requires  about  0.6 
parts  of  silica.  Where  the  retorted  metal  contains  a 
large  amount  of  iron,  sulphur  should  be  added  to  the  sur- 
face of  the  molten  metal  at  the  sides  of  the  melting  pot, 
and  stirred  in  with  a  plumbago  stirrer  to  form  a  matte  of 
sulphide  of  iron.  Theoretically  175  parts  of  iron  require 
100  of  sulphur. 

Should  the  amalgam  have  contained  some  sulphides,  the 
molten  metal  should  be  ' poled'  by  allowing  a  heated  iron 
rod  to  remain  in  the  pot  for  a  little  time  to  slag  off  the 
sulphur  as  iron-matte  (iron-sulphide).  If  the  amalgam 
contained  much  metallic  iron  this  'poling'  will  not  be  re- 
quired. If  the  quantity  of  iron  sulphide  formed  is  small, 
it  will  be  dissolved  by  an  excess  of  slag ;  if  large  it  will 
form  a  matte  between  the  bullion  and  slag.  This  matte 


126  STAMP  MILLING  AND  AMALGAMATION 

should  be  saved  and  after  a  quantity  from  several  melts 
is  collected,  should  be  fused  with  borax  and  soda  to  form 
a  button  of  gold  and  a  bar  of  clean  matte,  or  should  be 
cast  into  a  bar  and  shipped. 

Nitre  (potassium-nitrate)  is  used  to  oxidize  the  base 
metals  that  they  may  pass  into  the  slag,  but  it  also  oxidizes 
the  carbon  of  the  crucible,  corroding  it  badly,  consequently 
nitre  should  only  be  used  by  the  experienced  melter.  Sil- 
ica tends  to  increase  the  grade  of  the  bullion,  but  if  not 
used  in  the  right  proportion  the  slag  is  liable  to  become 
viscous  and  contain  shots  of  gold.  An  excess  of  soda 
makes  a  liquid  slag  and  one  that  separates  easily  from  the 
bar;  a  large  excess  can  easily  be  detected  in  cold  slag, 
especially  when  slacked  or  chilled  in  water,  from  hav- 
ing the  characteristics  of  soda.  An  excess  of  soda  will 
attack  the  crucible  while  an  excess  of  borax  will  not. 

For  melting  an  ordinary  retort  sponge  a  small  amount 
of  flux  consisting  of  two  or  three  parts  by  weight  of 
borax- glass  and  one  of  soda,  and  poling  with  an  iron  rod, 
if  the  amalgam  contained  much  sulphide,  is  all  that  will 
be  required  in  the  way  of  fluxing.  After  the  metal  and 
slag  have  subsided  to  quiet  fusion  the  mass  is  stirred  with 
an  iron  rod  that  has  been  previously  heated  red  hot  that 
no  gold  may  adhere,  the  object  being  to  settle  any 
shots  of  metal  in  the  slag  and  to  render  the  gold  homo- 
geneous. The  crucible  is  then  lifted  from  the  fire  by 
means  of  suitable  tongs  and  its  contents  are  poured  into 
an  iron  mould,  which  has  previously  been  well  smoked  in- 
side and  heated.  The  slag  rises  on  top  of  the  metal  and 
may  overflow  the  mould  without  doing  any  harm,  if  it  be 
quite  fluid.  It  is  improbable  that  any  shots  of  gold  that 
will  not  settle  while  in  the  furnace  will  do  so  after  pour- 


STICKING  IN  MOULDS  127 

ing,  so  all  slag  from  gold  melts  should  be  carefully  exam- 
ined for  shot  gold. 

The  mould  should  be  smooth  and  clean  on  the  inside,  all 
rust,  old  slag,  or  metal  should  be  removed.  It  should  be 
given  a  coating  on  the  inside,  preferably  of  carbon.  This 
may  consist  of  a  mixture  of  lampblack  and  lubricating  oil 
having  the  consistence  of  soft  butter.  Or  it  may  be  a 
coat  of  soot  given  by  inverting  the  mould  over  burning 
pitch  pine,  rosin,  or  oiled  waste  as  indicated  above.  White- 
wash can  be  used.  Thick  oil  has  been  used,  but  sputters 
while  pouring  and  afterward  burns  with  a  disagreeable 
smoke  and  odor.  The  purpose  of  this  coating  is  to  prevent 
the  gold  from  sticking  to  the  sides  of  the  mould  and  to  al- 
low the  bar  to  come  out  easily.  The  mould  should  be  well 
warmed,  but  not  excessively,  before  being  used,  that  it 
may  not  be  cracked  by  the  introduction  of  the  hot  metal 
and  that  the  gold  and  slag  may  not  be  suddenly  chilled,  in- 
terfering with  forming  a  neat  smooth  bar.  The  mould  is 
finally  leveled  that  the  bar  may  be  of  an  even  thickness. 
Usually  the  mould  is  placed  at  a  right  angle  to  the  flow 
from  the  melting  pot,  but  a  neater,  easier  pour  can  be  made 
by  setting  the  length  of  the  mould  parallel  to  the  flow. 
Greater  homogeneity  can  be  given  the  bar  by  continually 
moving  the  entering  stream  of  metal  up  and  down  the 
length  of  the  mould  in  pouring. 

After  the  gold  and  slag  have  cooled  sufficiently  to  be- 
come solid  they  are  dumped  out  of  the  mould  into  a  tub 
or  sink  of  water,  which  usually  causes  the  slag  to  separate 
easily  from  the  gold.  The  bar  is  cleaned  by  knocking 
and  scrubbing  off  any  bits  of  slag,  or  by  setting  back  in 
the  melting  pot  until  hot  and  then  plunging  it,  first  into 
dilute  sulphuric  acid,  and  then  into  water.  Nitric  acid  is 


128  STAMP  MILLING  AND  AMALGAMATION 

also  used.  If  the  bar  looks  very  base  and  dirty,  it  may 
be  re-melted  and  re-fluxed.  Two  opposite  corners  are 
chipped  off  for  assay,  or  it  is  bored  in  from  four  to  eight 
places  with  a  %-in.  drill,  rejecting  the  surface  borings; 
the  latter  method  of  sampling  is  to  be  preferred.  Some 
use  graphite  rods  for  stirring  the  molten  bullion,  these  are 
either  purchased  or  are  made  by  cutting  a  section  out  of 
an  old  or  condemned  crucible  in  the  shape  of  the  lower 
part  of  a  golf  club.  A  small  hole  is  bored  in  the  toe  of  the 
stirrer.  After  stirring  the  bullion,  the  gold  caught  in 
the  hole,  amounting  to  half  a  gram  or  more,  is  poured  into 
a  basin  of  water,  this  is  repeated  three  or  four  times  and 
the  bullion  assay  made  from  the  granules  obtained  in  this 
way.  A  dip  sample  taken  in  this  way  is  more  accurate 
than  any  bar  sample. 

The  slag  from  the  meltings,  likewise  old  melting  cru- 
cibles, are  saved ''and  eventually  run  through  the  clean-up 
barrel  in  a  separate  charge  to  recover  any  shots  of  gold. 
The  slag  can  be  sent  through  the  battery  if  there  is  no 
clean-up  barrel  available,  but  the  crucibles  should  first  be 
panned,  as  the  graphite  is  harmful  to  the  plate  amalgama- 
tion. After  this  treatment  the  tailing  should  be  assayed 
as  it  may  still  contain  sufficient  gold  to  warrant  shipping 
to  a  smelter. 

The  wood  removed  from  the  mortars,  together  with  old 
screen-frames,  and  all  wood  or  canvas  likely  to  contain 
any  amalgam  should  be  burned  and  the  ashes  put  through 
the  clean-up  barrel,  or  the  mortars,  in  lieu  of  a  barrel  or 
pan.  The  worn  out  screens  should  be  thoroughly  scrubbed 
and  pounded  after  being  taken  from  the  frames,  to  re- 
move any  amalgam,  and  then  placed  in  a  heap.  Shoes  and 
dies  and  pieces  of  iron  from  the  mortars  should  be 


CLEANING  SCREENS  129 

scrubbed  and  hammered  and  the  'eyes'  of  amalgam  in  the 
blow  holes  picked  out  by  means  of  old  round  files  tapered 
down  to  a  point,  finally  being  consigned  to  a  pile.  The  fine 
iron  removed  from  the  amalgam  should  be  placed  in  shal- 
low tubs.  The  oxidation  of  these  screens  and  coarse  and 
fine  iron  and  steel  should  be  promoted  by  occasionally  add- 
ing salt  and  frequently  wetting  with  water.  After  be- 
ing reduced  to  rust  as  far  as  possible,  that  material  which 
will  enter  the  clean-up  barrel  should  be  ground  up  in  it 
with  a  small  amount  of  mercury  and  finally  dropped  into 
water,  puddled,  and  the  finer  iron  removed  by  a  mag- 
net. It  may  be  necessary  to  re-wash  this  finer  material. 
The  screens  and  coarse  iron  receive  a  thorough  scrubbing 
and  pounding  before  being  finally  thrown  out.  Roasting 
or  burning  the  screens  and  fine  iron  is  a  quick  way  to 
loosen  the  adhering  amalgam  and  to  promote  oxidation. 


Part  III 

GENERAL 
CHAPTER  IX 

The  loss  of  gold  in  amalgamation  may  be  due  to : 

(1)  Free  gold  included  in  or  surrounded  by  the  gangue 
rock. 

(2)  Gold  that  is  free  and  chemically  combined  in  the 
sulphides  and  tellurides. 

(3)  'Float'  gold  that  is  carried  along  on  top  of  or  sus- 
pended in  the  pulp  and  which  does  not  come  in  contact 
with  the  amalgamated  plate. 

(4)  'Rusty'  or  coated  gold. 

(5)  '  Overstamping. ' 

(6)  Poor  amalgamation  due  to  the  methods  in  use. 

(7)  Poor  amalgamation  due  to  deleterious  substances 
in  the  ore. 

First :  If  the  loss  is  in  the  free  gold  included  in  or  sur- 
rounded by  the  gangue  rock,  a  sizing  test  will  reveal  it  by 
the  higher  value  of  the  coarser  sands.  In  some  cases  the 
coarser  sands  can  be  crushed  finer  in  a  hand  mortar  and 
panned  to  show  a  'prospect*  of  free  gold.  The  correction 
for  this  is  to  crush  finer,  not  the  ore  in  general,  but  these 
coarser  grains.  This  crushing  is  better  accomplished  by 
using  a  finer  screen  with  the  same  or  a  lower  height  of  dis- 
charge. Running  two  batteries  with  different  size  screens 
in  competition  with  each  other  and  comparing  the  tailing 
assays  will  determine  in  an  empirical  way,  but  sizing  tests 


GOLD  IN  SULPHIDES  131 

in  connection  with  this  is  necessary  for  a  true  diagnosis  of 
the  ore. 

Second:  Gold  in  association  with  tellurium  is  chem- 
ically combined  and  can  only  be  saved  by  cyanidation 
along  special  lines,  by  chlorination,  or  by  smelting,  and 
rarely  by  concentration. 

Gold  in  the  sulphides  is  mainly  in  a  free  state,  finely 
divided,  and  mechanically  held  by  the  sulphides.  This 
gold  is  usually  saved  by  concentration,  and  in  some  cases 
by  cyanidation  of  the  tailing  without  concentration.  How- 
ever, a  part  of  it  can  be  amalgamated,  as  in  the  Gilpin 
County  practice,  by  the  use  of  a  wide  mortar,  deep  dis- 
charge, long  and  slow  drop,  and  an  attempt  to  catch  the 
gold  inside  the  mortar.  Here  the  sulphide  because  of  its 
higher  specific  gravity  sinks  to  the  bottom  and  is  held 
longer  in  the  mortar  than  the  balance  of  the  pulp,  allow- 
ing the  gold  to  be  liberated  by  the  thorough  sliming  of 
the  sulphide,  and  to  be  brought  in  long  contact  with  the 
mercury  and  inside  plates.  This  process  has  had  slight 
application  outside  of  the  locality  mentioned,  where  it  was 
necessitated  by  a  large  proportion  of  the  gold  being  con- 
tained in  the  sulphide  that  was  of  too  low  a  grade  to  ship 
and  smelt  profitably.  Part  of  the  loss  in  the  tailing  may 
be  due  to  sulphide  crushed  so  fine  (slimed),  that  it  cannot 
be  caught  on  the  concentrators ;  this  will  be  considered  un- 
der 'Overstamping. '  The  amalgamation  of  the  gold  in  the 
sulphides  has  been  accomplished  by  grinding  them  in  amal- 
gamating pans  or  arrastres,  but  the  extraction  has  never 
been  high,  so  that  it  is  now  preferable  to  cyanide  them  if 
they  contain  no  interfering  elements,  or  to  ship  them  to 
the  smelters. 

Third:     'Float'  gold  really  refers  to  that  gold  which 


132  STAMP  MILLING  AND  AMALGAMATION 

occurs  in  flakes  so  light  and  thin  that  it  is  floated  along 
on  the  surface  of  the  pulp,  perhaps  buoyed  up  by  a  btfbble 
of  air,  but  in  most  cases  it  will  be  found  to  be  gold  so  fine 
that  it  is  carried  suspended  in  the  pulp  and  gets  no  oppor- 
tunity to  come  in  contact  with  the  amalgamated  plate. 
When  an  ore  containing  visible  gold  is  pounded  up  in  a 
hand  mortar  and  panned,  the  gold  is  found  to  be  pounded 
into  scales  or  into  infinitesimal  bits,  depending  on  the  na- 
ture of  the  gold.  It  is  doubtful  if  much  gold  is  over- 
stamped  to  an  extent  producing  scales  so  thin  that  they 
will  float  on  the  surface  of  water  like  gold-leaf,  although 
such  gold  has  been  found  in  both  mills  and  placers;  but 
it  can  be  understood  that  gold  which  is  powdered  fine  may 
be  carried  along  in  the  pulp  clear  of  the  plates,  though  it 
really  does  not  float.  The  loss  attributed  in  a  tentative 
way  to  float  gold  is  found  by  assaying  the  flocculent  slime 
of  the  tailing.  Should  this  show  a  value  as  high  or  higher 
than  the  sand,  it  would  indicate  overstamping,  and  ad- 
justments calculated  to  prevent  this  should  be  made.  A 
part  of  the  value  lost  in  the  tailing  and  assigned  to  float 
gold  is  in  the  slimed  sulphide,  but  only  a  part  of  the  loss 
can  be  rightfully  ascribed  to  this.  Increasing  the  grade 
of  the  plates  and  using  as  little  water  as  possible  in  the 
mortar  to  secure  a  better  wave  motion  and  contact  be- 
tween the  pulp  and  the  plates,  together  with  making  the 
plates  longer  and  keeping  them  covered  with  a  bed  of 
soft  amalgam,  will  aid  in  saving  more  of  the  fine  and 
float  gold.  If  the  battery  water  is  being  re-used  it  should 
be  well  settled,  for  as  the  water  or  pulp  becomes  thicker 
and  more  slimy  it  will  carry  off  more  of  this  light  gold. 
Patent  amalgamators  for  catching  this  kind  of  gold  should 
be  tried.  It  is  practically  impossible  to  determine  the 


RUSTY  GOLD  133 

form  or  condition  of  the  gold  in  these  slimes  where  the 
amalgamation  and  concentration  has  been  capably  done, 
or  to  hope  to  promote  any  further  extraction  by  labora- 
tory and  amalgamation  tests. 

Fourth:  'Rusty'  gold  is  free  gold  covered  with  a  film 
of  some  substance  other  than  air  or  the  gangue  rock  in 
which  it  is  contained.  This  may  be  due  to  an  oily  or 
greasy  mineral  peculiar  to  the  ore,  like  graphite;  to  an 
oxide  of  iron  or  copper,  or  to  other  compounds  of  the  base 
metals;  to  silicates  of  magnesia  or  alumina,  or  to  slime 
arising  from  crushing  the  ore.  This  film  prevents  the 
gold  from  coming  in  direct  contact  with  the  mercury. 
Rusty  gold  can  sometimes  be  detected  by  panning  the  tail- 
ing and  examining  the  concentrate  with  a  microscope. 
This  gold  should  be  caught  by  the  concentrators,  or  in  the 
cyanide  plant  if  not  too  coarse,  also  by  the  use  of  riffles 
or  blanket  tables.  To  amalgamate  this  gold  it  must  be 
scoured.  This  can  be  done  by  using  a  high  discharge,  pre- 
ferably with  a  coarser  screen  and  a  narrow  mortar  that 
the  tendency  to  overstamp  may  be  reduced.  With  a  low- 
discharge,  rapid-crushing  mortar,  the  ore  is  in  the  mortar 
an  average  of  four  or  five  minutes,  this  length  of  time  can 
be  doubled  or  trebled  by  increasing  the  height  of  dis- 
charge, so  that  the  particles  of  gold,  especially  the  heavier 
ones,  are  subjected  to  the  scouring  and  attrition  of  the 
stamp  and  the  pulp  for  an  increased  length  of  time.  So- 
dium amalgam  in  the  mercury  should  be  tried.  If  any  of 
this  gold  is  retained  in  the  traps,  they  should  be  cleaned 
often,  perhaps  at  each  plate  dressing,  the  sand  being 
ground  in  the  clean-up  barrel  with  some  additional  mer- 
cury, to  scour  and  amalgamate  this  gold.  Theoretically, 
coarse  crushing  in  the  mortar  followed  by  regrinding 


134  STAMP  MILLING  AND  AMALGAMATION 

and  amalgamating  in  a  pan  or  roller  mill  should  be  suc- 
cessful. It  is  considered  that  the  pan  amalgamator  will 
amalgamate  gold  that  no  other  method  of  amalgamation 
will  save. 

Fifth :  'Overstamping'  is  holding  the  pulp  longer  in  the 
mortar  subject  to  the  action  of  the  stamps  than  is  neces- 
sary, thereby  pulverizing  it  finer  than  required  or  than  is 
beneficial.  While  the  capacity  is  reduced,  the  term  prop- 
erly has  no  reference  to  that,  but  to  the  treatment  the  ore 
receives  causing  it  to  give  a  reduced  extraction.  Experi- 
ments have  shown  that  a  hammered  gold  is  not  readily 
amalgamable,  which  further  experiments  tend  to  prove  to 
be  due  to  the  gold  being  covered  with  a  film  of  dirt  and 
grease  in  the  process  of  hammering,  which  in  connection 
with  its  increased  density,  does  not  allow  it  to  be  so  easily 
wetted  by  the  mercury.  As  has  been  observed  under  float 
gold,  it  is  improbable  that  much  gold  which  can  be  ham- 
mered into  a  scale  is  rendered  non-amalgamable  by  stamp- 
ing in  the  mortar,  especially  in  the  presence  of  mercury; 
while  there  is  no  doubt  that  gold  in  the  allotropic  form 
of  being  brittle  can  be  stamped  so  fine  that  it  is  hard  to 
catch  in  the  mortar  or  on  the  plates,  particularly  if  it  is 
covered  with  slime.  The  danger  of  overstamping  is  aug- 
mented with  increase  in  the  grade  and  percentage  of  the 
sulphide.  The  Gilpin  County  practice  is  an  ideal  illustra- 
tion of  overstamping  sulphide. 

As  to  whether  the  ore  is  being  overstamped  or  not  is 
judged  from  the  sizing-test  assays  taken  in  connection  with 
the  tonnage  and  operating  expenses.  If  the  assays  of  the 
slime  and  fine  sands  closely  approach  or  are  higher  than 
those  of  the  coarser  sands,  adjustments  should  be  made 
that  will  reduce  the  percentage  of  fine  material  in  favor 


POOR  AMALGAMATION  135 

of  a  higher  tonnage.  If  it  is  an  actual  case  of  overstamp- 
ing,  resulting  in  the  gold  being  rendered  less  amalgam- 
able  by  being  hampered,  broken  up,  or  coated  with  a 
film,  or  to  the  sulphide  being  slimed,  the  proper  change 
of  adjustments  should  reduce  the  assays  of  the  slime  and 
finer  sands.  The  changes  of  adjustment  have  one  object 
in  view — to  get  the  pulp  out  of  the  mortar  as  quickly  as 
possible  after  having  been  crushed  to  the  proper  size  to 
liberate  the  gold  and  sulphide  from  their  matrix. 

Sixth :  Poor  amalgamation,  due  to  the  methods  in  use, 
may  keep  the  amalgam  so  hard  that  it  becomes  crystalline 
and  breaks  away,  or  that  it  reduces  the  tendency  of  the 
gold  to  catch ;  in  keeping  the  plates  so  wet  that  the  mer- 
cury and  amalgam  run  down  into  the  trap ;  to  the  gold  not 
catching  due  to  stains,  bare  spots,  plates  cleaned  too  close, 
too  much  water  used,  too  small  a  plate  area ;  loss  of  amal- 
gam by  not  removing  or  bedding  down  the  crumbs  when 
dressing  the  plates;  use  of  impure  mercury;  grease  fall- 
ing into  the  mortar  and  contaminating  the  plates;  in 
fact,  to  bad  practice  in  any  of  the  various  details  con- 
nected with  amalgamating. 

Seventh:  Poor  amalgamation  due  to  deleterious  sub- 
stances in  the  ore  does  not  often  occur.  Arsenical  and 
antimonial  ores  are  the  worst  offenders  in  this  regard. 
They  tend  to  foul  the  mercury  and  amalgam,  coating  it 
with  a  film  of  the  slimed  material  so  that  the  mercury 
does  not  readily  amalgamate  with  the  gold,  but  is  sick- 
ened and  a  large  part  of  it  lost.  This  trouble  is  liable  to 
occur  to  some  extent  with  any  base  and  heavily  sulphur- 
etted ore.  The  principal  remedy  is  to  practise  outside 
amalgamation.  Clayey,  talcose,  and  other  slimy  ores 


136  STAMP  MILLING  AND  AMALGAMATION 

sometimes  give  trouble  in  a  similar  way,  or  by  coating 
the  gold. 

In  making  mill  tests  two  batteries  should  be  selected 
that  receive  a  feed  as  nearly  identical  as  possible.  Com- 
parative tests  should  be  made  simultaneously  with  the 
different  adjustments.  Sizing  of  the  tailing  samples  from 
these  two  batteries  should  exhibit  the  characteristics  of 
the  ore  under  the  different  treatments.  These  tests  may 
be  conducted  in  the  following  manner.  Each  tailing  sam- 
ple, understood  to  have  been  carefully  taken  and  in  every 
way  representative,  is  drained  of  its  settled  water,  dried 
and  thoroughly  mixed.  A  quantity  of  from  30  to  60  oz. 
is  removed  for  the  sizing  test,  also  a  'head'  assay  sam- 
ple taken.  All  assays  should  be  made  in  duplicate.  The 
sizing  sample  is  panned  and  repanned  until  all  the  con- 
centrate is  removed,  this  concentrate  to  be  examined  for 
rusty  gold  and  amalgam.  The  sample  together  with  the 
water  used  in  panning  is  now  thoroughly  stirred  and 
after  settling  for  a  moment,  the  slimy  water  is  poured 
off,  care  being  exercised  that  no  sand  passes  over  with 
it.  More  water  is  added  to  the  sand  and  the  process  re- 
peated again  and  again  until  only  the  granular  sand  re- 
mains and  the  water  contains  slime  that  is  a  true  floccu- 
lent  slime,  is  light  and  feathery,  with  agglomerates,  does 
not  readily  settle  in  water,  and  which  makes  water  muddy, 
in  contra-distinction  to  sharp,  granular  sand  which  readily 
settles  and  does  not  make  water  muddy.  The  sand  is 
sized,  either  before  or  after  drying,  into  a  coarse,  medium, 
and  fine  sand.  Where  comparative  tests  are  not  being 
made  on  two  batteries,  a  fourth  size,  an  extra  coarse  sand, 
should  be  made.  Thus,  when  crushing  through  a  30-mesh 
screen,  by  making  an  extra  coarse  size  out  of  that  held 


MILL  TESTS 


137 


on  a  40-mesh  screen,  we  may  be  able  to  judge  how  the  ore 
passing  it  will  act  when  crushed  through  a  40-mesh 
screen.  The  sand,  slime,  and  concentrate  are  now  dried, 
weighed,  and  assayed,  after  which  -the  results  may  be 
tabulated.  The  following  is  taken  from  a  note  book,  and 
is  an  actual  test  made  on  a  small  lot  of  tailing  that  was 
dried  before  being  weighed  for  the  sizing  test : 


MILL  Tesr.     G  A//M 

No.  v5tf  Brass  W/re  Sc/ 
v5"  /ft  c/t    DiscJi  a  rye  . 

>//»  acf    Stx^ay               ,   #  /^  v 

£.       Nov.  7,  W. 

*ee/i. 

30  . 
Grams. 

Weight  -taken  600 

3/Z£ 

IYE.  /  GH  T 

ASSAY 
V/1LUZ 

P£PA. 

r/u-uc 

GRAMS 

c/o 

He/e/  on  4-O  fries  h 

80 

/3.4 

1.40 

0.19 

t       #     €o         tf 

33 

/J.7 

1.4-0 

0.2Z 

if       -     IOO          » 

77 

/s3. 

/.+  0 

O.t& 

Passec/  SOO         " 

/  /2 

/  8.9 

/.20 

O.Z3 

F/occulent   3//rrie 

Z3Z 

39. 

/.CO 

0.39 

Cone  en  +r~ai-€> 

A/0/7C 

foutic\ 

t 

*594 

too 

J.2I 

The  deductions  from  the  above  test  are  that  the  con- 
centration is  nearly  perfect,  but  that  the  ore  is  being 
crushed  too  fine  for  the  purpose  of  economic  amalgama- 
tion, since  the  assay  of  the  slime  is  comparatively  high 
and  the  quantity  abnormally  large,  while  the  coarse  sand, 
that  held  on  '40-mesh/  assays  no  more  than  the  finer  sand. 


138  STAMP  MILLING  AND  AMALGAMATION 

The  results  of  a  single  sizing  test  should  not  be  taken 
as  conclusive,  but  a  series  made.  The  capacity  may  be 
obtained  by  catching  the  pulp  in  a  tub  or  barrel  for  a 
certain  length  of  time  and  weighing  the  dry  pulp.  Where 
cyanidation  follows  amalgamation  and  concentration,  the 
adjustments  of  the  battery  will  be  determined  by  the 
sizing  tests  of  the  cyanided  tailing,  which  indicates  how 
fine  the  ore  should  be  crushed  to  get  the  maximum  ex- 
traction by  the  cyanide  solution. 

Generally  too  little  attention  is  given  to  testing  and 
studying  an  ore  before  erecting  a  mill,  though  the  stamp- 
battery,  thanks  to  its  wide  range  of  adaptability,  can 
usually  be  made  to  do  satisfactory  work,  consequently  the 
change,  if  any,  usually  takes  place  in  the  concentrating 
and  cyaniding  departments.  The  usual  procedure  is  to 
take  a  sample  of  the  ore  which  is  seldom  representative 
of  the  run-of-mine  ore.  This  ore  will  probably  come  from 
a  dump,  or  near  the  surface,  and  be  an  oxidized  ore,  while 
the  assay  value  will  be  high.  After  making  a  trial  run  at 
a  testing  works,  a  mill  will  be  ordered  by  the  directors  of 
the  company,  the  details  of  the  mill  being  left  to  the  ma- 
chinery supply  house.  The  mill-site  may  be  selected  and 
the  mill  designed  by  a  man  who  has  had  little  or  no  experi- 
ence in  milling.  Finally,  the  mill  is  completed  and  turned 
over  to  the  millman  who  must  then  spend  considerable 
time  in  changing  and  rearranging.  It  is  incomprehensible 
why  mining  companies  so  seldom  employ  competent  metal- 
lurgists, independent  of  machinery  supply  houses  and 
special  process  companies,  to  examine  the  ores  of  their 
mines  and  to  design  and  build  a  mill  suited  to  those  par- 
ticular ores  and  conditions.  The  cost  and  loss  of  time 
in  changing,  rearranging,  and  providing  for  those  things 


AMALGAMATION  TESTS  139 

that  have  been  overlooked  would  pay  for  the  metallurgist, 
to  say  nothing  of  the  daily  saving  that  may  be  effected  in 
a  properly  designed  mill.  Such  a  man  should  more  than 
save  the  expense  of  his  fee  by  knowing  what,  how,  and 
where  to  buy. 

The  metallurgist  should  himself  take  representative 
samples  of  the  different  ores  and  make  laboratory  tests  in 
amalgamation,  concentration,  and  cyanidation,  together 
with  sizing  tests,  that  he  may  thoroughly  understand  the 
ore.  For  making  amalgamation  tests,  twe  methods  can 
be  followed.  The  first  is  to  place  six  or  eight  assay  tons 
of  the  ore  crushed  to  the  desired  mesh  in  a  large  glass 
bottle  with  sufficient  water  to  make  a  thin  pulp,  adding 
l/2  oz.  of  mercury.  The  pulp  is  rolled  around  in  the  bottle, 
is  lightly  shaken,  and  is  given  a  panning  motion  for  some 
time,  that  all  the  free  gold  may  be  amalgamated.  The 
contents  are  finally  washed  out  of  the  bottle,  panned  and 
repanned  until  the  amalgam  is  separated  from  the  pulp, 
when  the  tailing  is  dried  and  assayed;  the  difference  be- 
tween the  head  and  tailing  assay  representing  the  amount 
amalgamated.  If  mercury  that  contains  no  gold  has  been 
used  in  this  test,  the  gold  in  the  amalgam  can  be  deter- 
mined and  the  amount  amalgamated  ascertained  in  this 
way.  The  amount  of  gold  is  found  by  boiling  the  amalgam 
in  dilute  nitric  acid  until  only  the  pure  gold  remains, 
when  it  can  be  washed,  dried,  annealed,  and  weighed  as 
usual  in  the  gold  assay;  or  the  amalgam  may  have  the 
mercury  driven  off  by  heating  it  in  the  open  where  there 
is  no  danger  of  salivation,  and  cupelling  the  resulting 
sponge.  Mercury  entirely  free  from  gold  can  seldom  be 
obtained,  but  can  easily  be  prepared  by  dissolving  it  in 
dilute  nitric  acid,  when  the  gold  remains  undissolved  and 


140  STAMP  MILLING  AND  AMALGAMATION 

can  be  filtered  off,  while  the  mercury  can  be  precipitated 
by  suspending  a  piece  of  copper  in  the  solution. 

A  better  method  of  making  an  amalgamation  test  is  to 
work  the  ore  as  a  thin  pulp  in  a  gold-pan  having  an 
amalgamated  bottom,  assaying  before  and  after  treat- 
ment; the  pan  being  used  to  separate  any  sulphide  pres- 
ent at  the  same  time.  Laboratory  amalgamation  tests,  as 
a  rule,  will  not  give  as  high  an  extraction  as  will  be  ob- 
tained in  actual  mill  practice.  This  may  be  due  to  the 
fact  that  in  preparing  ore  for  such  a  test,  it  is  screened 
frequently,  resulting  in  an  evenly  sized  material,  whereas 
in  actual  practice  a  large  proportion  is  crushed  much  finer 
and  should  give  higher  extraction.  It  is  also  possible  that 
the  dry  crushing  may  coat  the  gold  with  dirt  or  slime  so 
that  to  some  extent  it  resists  amalgamation.  ^ 

The  points  it  will  be  necessary  for  the  metallurgist  to 
know  are:  what  percentage  of  the  gold  will  amalgamate 
under  ordinary  crushing,  and  what  increased  extraction 
can  be  obtained  by  regrinding  and  amalgamating  a  sec- 
ond time;  what  percentage  of  concentrate  there  is  in  the 
ore,  its  nature,  value  per  ton,  and  amenability  to  cyanide 
or  other  treatment ;  what  extraction  can  be  secured  from 
the  ore  on  the  plates,  on  the  concentrators,  and  from  the 
tailing  by  cyaniding  with  coarse,  medium,  and  sliming 
crushing;  and  the  nature,  occurrence,  and  condition 
of  the  gold  in  the  ore.  Making  tests  with  a  few  pounds 
of  ore  has  been  decried,  but  while  such  small  tests 
are  not  conclusive,  they  are  a  reliable  guide  when 
the  sample  represents  the  ore  fairly  and  the  tests  are  con- 
ducted by  a  metallurgist  experienced  in  tLe  processes. 
They  will  enable  a  system  of  treatment  to  be  outlined 
that  should  be  tried  out  by  treating  a  few  tons  of  the 


MILL  SAMPLES  141 

different  classes  of  ore  at  a  testing  works  under  the  per- 
sonal direction  of  the  metallurgist.  The  extent  to  which 
the  testing  should  be  carried  varies  with  the  nature  of  the 
ore  and  largely  to  the  degree  to  which  the  metallurgy  of 
similar  ores  in  that  locality  has  been  worked  out.  There 
are  some  districts  where  it  seems  hardly  necessary  to  test 
the  ore  for  a  process,  such  as  the  Mother  Lode  of  Cali- 
fornia. There  are  other  districts,  however,  where  the 
working  out  of  a  successful  treatment  system  seems  to 
almost  require  a  full-size  mill  operating  under  the  actual 
working  conditions,  such  as  was  seen  in  the  case  of  the 
silver  ores  of  the  Tonopah  district  of  Nevada. 

The  taking  of  mill  samples  should  be  done  as  auto- 
matically as  possible.  A  sample  of  the  battery  feed  taken 
by  picking  it  from  the  revolving  plate  of  the  feeder  by 
hand  is  absolutely  unreliable  on  account  of.  taking  too 
large  a  proportion  of  the  coarse  ore.  The  sizing  of  a  bat- 
tery-feed sample  through  a  quarter  or  half-inch-mesh 
screen  will  usually  show  that  the  fine  material  assays  three 
or  four  times  as  much  as  the  coarse,  and  may,  in  some 
rare  instances,  show  the  reverse.  A  long  tin  trough  or 
scoop  that  can  be  placed  beneath  the  revolving  plate  and 
catch  all  of  the  ore  as  it  drops  from  the  feeder  will  give 
a  much  better  sample  when  taken  hourly  or  half-hourly, 
especially  if  the  fine  and  coarse  ore  is  well  mixed  in  the 
bin.  Where  outside  amalgamation  is  practised,  the  sam- 
ple of  the  battery  feed  is  obtained  in  front  of  the  mortar 
just  before  the  pulp  strikes  the  plate. 

The  tailing  sample  should  be  taken  automatically  by  a 
device  for  that  purpose,  and  the  millman  should  be  under 
instructions  not  to  put  it  out  of  use  during  the  period  of 
dressing  the  plates.  The  millman  fears  that  loose  amal- 


142  STAMP  MILLING  AND  AMALGAMATION 

gam  will  be  washed  away  to  'salt'  the  sample;  but  if 
amalgam  to  this  extent  is  being  lost,  immediate  steps  to 
prevent  it  should  be  taken.  Where  the  samples  are  taken 
by  hand  they  are  liable  to  become  'picked'  samples,  as, 
for  instance,  where  the  concentrator  man  carefully  ad- 
justs each  machine  before  taking  the  tailing  sample.  For 
catching  the  tailing  sample  from  an  automatic  sampler, 
the  usual  gasoline  can  may  be  used  with  a  light  tin  pipe 
3  or  4  inches  in  diameter  fastened  to  a  few  pieces  of  wood 
which  allow  it  to  rest  on  the  can  with  its  bottom  end  pro- 
jecting into  the  can.  The  pulp  flows  into  the  can  through 
this  pipe,  and  when  the  can  is  full  the  clear  water  com- 
mences to  overflow  without  any  attention  from  the  man 
in  charge,  who  finds  the  pulp  well  settled  when  he  comes 
to  remove  the  sample. 

Where  the  gold  will  amalgamate  to  the  extent  of  20% 
or  more,  amalgamation,  preferably  in  water,  may  be 
practised.  If  the  ore  will  not  amalgamate  to  this  extent 
and  requires  cyanide  treatment,  amalgamation  had  better 
be  dispensed  with  unless  some  of  the  amalgamable  gold  is 
coarse  and  escapes  the  cyanide  plant.  Crushing  in  cyanide 
solution  is  a  great  aid  to  cyaniding,  as  the  ore  is  brought 
promptly  into  contact  with  the  solution,  and  under  con- 
ditions that  cause  the  value  to  go  into  solution  quickly, 
thus  requiring  less  tankage  for  dissolving  the  gold.  It 
also  permits  using  a  certain  amount  of  water  in  washing 
the  dissolved  value  out  of  the  pulp  to  compensate  for  the 
moisture  discharged  in  the  tailing.  This  makes  a  saving 
in  the  amount  of  cyanide  mechanically  lost,  and  in  prac- 
tice also  effects  a  saving  in  the  amount  of  dissolved  gold 
mechanically  lost.  Crushing  in  solution  has  its  disad- 
vantages in  that  the  solution  throughout  the  mill  is  car- 


CRUSHING  IN  SOLUTION  143 

rying  quite  an  amount  of  gold;  and  a  little  of  this  solu- 
tion is  constantly  being  lost,  even  in  well-designed  mills, 
by  the  leakages,  overflows,  accidents,  and  otherwise. 
Another  disadvantage  of  crushing  in  cyanide  solution  in 
a  mill  where  amalgamation  is  being  practised  is  that  a 
part  of  the  gold  goes  into  solution  which  would  be  amal- 
gamated if  the  crushing  were  done  in  water.  If  this  gold 
were  amalgamated,  practically  all  of  it  would  be  returned, 
but  by  going  into  solution  the  proportion  returned  is 
lessened,  both  through  the  losses  above  referred  to,  and 
through  the  indifferent  washings  of  the  filtering  devices 
used.  The  amount  of  gold  going  into  solution  that  could 
otherwise  be  obtained  by  amalgamating  in  water  will  vary 
with  the  size  of  the  particles  of  gold;  thus  with  an  ex- 
tremely fine  gold  crushed  in  strong  cyanide  solution  as 
much  as  one-third  or  one-half  of  the  amalgamable  gold 
may  be  dissolved  so  quickly  that  it  cannot  be  amalgam- 
ated. In  such  a  case  it  is  inadvisable  to  crush  in  cyanide 
solution.  Where  the  gold  is  coarse  and  but  little  of  the 
amalgamable  gold  enters  the  solution,  crushing  in  solu- 
tion may  be  advisable,  depending  on  the  character  of  the 
cyanide  plant  and  the  perfection  of  its  operation  in 
gathering  all  of  the  dissolvable  value  into  the  clean-up. 

Where  the  sulphide  is  amenable  to  cyanide  treatment 
and  is  small  in  amount  or  low  in  value,  it  may  be  ex- 
pedient to  cyanide  with  the  sand  without  concentration, 
offsetting  the  decreased  extraction  from  this  sulphide  by 
the  lessened  cost  of  treatment.  At  one  prominent  prop- 
erty operating  along  this  line,  a  high  discharge  is  used  to 
retain  the  sulphide  longer  in  the  mortar,  that  it  may  give 
a  higher  extraction  in  the  cyanide  plant,  through  being 


144  STAMP  MILLING  AND  AMALGAMATION 

crushed  fine,  while  a  large  amount  of  water  is  used  in  the 
mortar  to  increase  the  tonnage. 

Economic  problems  must  be  studied  when  considering 
amalgamation  and  cyanidation.  With  a  small  mill  of  10 
or  20  stamps  obtaining  a  good  extraction  by  amalgama- 
tion and  concentration,  it  may  not  be  advisable  to  put  in 
a  cyanide  plant  taking  the  pulp  directly,  on  account  of 
the  high  cost  per  ton  of  capacity  for  installing  and  op- 
erating a  small  plant  of  this  type.  It  may  be  better  to 
run  the  tailing  into  a  pond  and  later  put  in,  at  less  ton- 
nage expense,  a  leaching  plant  of  large  capacity.  With  a 
pulp  crushed  through  a  30  or  40-mesh  screen  and  properly 
impounded,  it  is  possible  to  extract  practically  all  of  the 
dissolvable  value.  In  a  country  of  average  working  costs, 
an  impounded  tailing  having  a  value  of  80  cents  per  ton 
and  giving  an  extraction  of  80%  will  return  a  good  mar- 
gin of  profit.  As  the  plate  or  concentrator  tailing  be- 
comes higher,  the  necessity  for  a  plant  to  treat  it  directly 
increases,  since  the  tailing  pond  will  hold  a  large  amount 
of  money  that  is  not  available,  and  there  is  a  loss  from 
the  sand  blown  away  and  an  occasional  breaking  of  the 
dam.  A  tailing  pond  is  sometimes  a  desirable  thing  to  a 
manager,  or  promoter,  as  when  it  figures  prominently,  too 
prominently  usually,  in  the  report  of  the  assets  and  pos- 
sibilities of  the  company's  property.  Regrinding  of  the 
pulp  followed  by  amalgamation  (secondary  amalgama- 
tion) may  reduce  the  value  of  the  tailing  to  a  point  so 
low  that  it  may  not  be  profitable  to  cyanide  it.  For  this 
purpose  some  form  of  grinding  pan,  a  Chilean  mill,  or  the 
slow-speed  roller  mill,  that  will  admit  of  amalgamation 
within  the  mill,  may  be  recommended.  The  tube-mill  has 
been  found  the  most  satisfactory  machine  for  fine  grind- 


ALL  SLIMING  145 

ing  for  cyanidation.  It  has  generally  been  considered  a 
poor  machine  for  amalgamating  purposes,  since  any  mer- 
cury fed  to  it,  as  well  as  the  gold  which  is  liberated,  is 
supposed  to  come  out  thoroughly  slimed.  However,  ex- 
cellent outside  amalgamation  can  be  effected  after  the 
tube-mill,  as,  due  to  the  fineness  of  the  pulp,  a  beautifully 
thin  flow  can  be  had  over  the  plates.  When  crushing  in 
cyanide  solution  the  grinding  and  sliming  action  within 
the  mill  causes  so  much  of  the  gold  to  go  into  solution 
that  it  is  usually  not  worth  while  to  try  amalgamation 
afterward. 

From  a  theoretical  standpoint  it  would  appear  that 
where  amalgamation  or  concentration  is  to  follow  the 
tube-mill,  the  mill  should  be  run  at  a  speed  that  will  cause 
the  balls  to  be  carried  part  way  around  and  to  crush  by 
their  falling  impact,  rather  than  the  slower  rate  of  speed 
whereby  comminution  is  effected  by  the  attrition  or  rolling 
and  grinding  of  the  pebbles  alone.  The  first  is  a  case  of 
cracking  open  the  grains  and  liberating  the  gold,  or  sul- 
phide, as  a  relatively  large  angular  particle  susceptible 
of  easy  amalgamation,  or  concentration,  whereas  the 
second  results  in  the  scaly,  impalpable,  unmanageable 
slime  produced  by  attrition.  The  first  is  illustrative  of 
the  principle  of  the  stamp,  the  second  that  of  the  grinding 
mill.  An  appreciation  of  these  principles  will  lead  to  a 
better  understanding  of  the  reason  for  the  supremacy  of 
the  stamp-mill. 

Within  the  past  few  years  fine-grinding  and  'all- 
sliming,  '  invariably  connected  with  'crushing  in  solution, 
has  rapidly  come  into  vogue.  In  most  cases  it  has  been 
advisable,  but  in  many  instances  these  methods  have  been 
employed  because  it  is  the  fad,  or  because  filtering  de- 


146  STAMP  MILLING  AND  AMALGAMATION 

vices  were  installed  that  required  them.  This  is  clearly 
wrong.  Fine-grinding  should  not  be  carried  beyond  the 
economic  point.  The  cost  of  finer  comminution  increases 
rapidly,  whether  by  stamp,  Chilean  mill,  tube-mill,  or  other 
device.  The  degree  of  fineness  that  will  give  the  highest 
extraction  in  the  laboratory  is  not  necessarily  that  which 
will  give  the  most  profit.  The  milling  and  cyaniding  ma- 
chinery should  be  susceptible  of  adaptation  to  the  econ- 
omic requirements,  and  the  millman  or  metallurgist  should 
possess  the  ability  to  find  them. 

It  may  be  given  as  a  rule  of  broad  application  that  the 
higher  the  grade  and  the  baser  the  ore,  the  better  adapted 
it  is  for  all-sliming  and  crushing  in  solution,  while  the 
lower  the  grade  and  the  less  base  it  is,  the  less  adapted  it 
will  be  to  these  methods,  since  their  cost  hinges  mainly 
on  the  degree  of  fineness  and  their  application  generally. 
While  the  percentage  of  additional  extraction  made  by 
these  methods  usually  increases  with  the  grade  of  the  ore, 
yet  even  in  those  cases  where  it  may  be  essentially  the 
same,  on  either  a  high  or  a  low-grade  ore,  with  a  lessening 
grade,  a  point  is  soon  reached  where  the  additional  ex- 
traction will  not  pay  the  increased  cost. 

It  is  a  fact  that  crushing  in  solution  has  been  generally 
unsatisfactory  on  low-grade  ore,  and  there  do  not  appear 
to  be  any  plants  operating  under  such  conditions  today — 
or  at  least  any  that  are  widely  known  outside  of  the  Black 
Hills — yet  there  are  many  plants  employing  final  cyanida- 
tion  on  low-grade  ore.  The  question  now  comes,  if  crush- 
ing in  solution  has  to  give  way  to  other  methods  on  low- 
grade  ore,  why  are  not  these  other  methods  more  econ- 
omical on  higher  grade  ore?  Various  factors  enter  into 
the  consideration  of  this  matter,  but  the  principal  reason 


NOVELTY  MILLS  14? 

why  no  satisfactory  answer  has  yet  been  made  appears  to 
be  that  the  metallurgists  who  should  give  us  the  answer 
are  too  busy  boosting  special  processes  in  which  they  are 
interested. 

There  have  been  a  number  of  mills  built  to  employ  these 
methods,  which  have  not  proved  successful.  These  have 
usually  been  small  mills,  either  not  elaborately  and  care- 
fully designed  or  embodying  some  rather  new  and  untried 
devices.  The  best  way  to  handle  these  ' novelty  mills'  is 
to  go  back  to  proved  methods.  Crush  in  water  and  amal- 
gamate, following  with  regrinding  and  secondary  amal- 
gamation. Get  all  that  can  possibly  be  obtained  by  amal- 
gamation, for  that  will  be  '  absolute, '  at  least  in  so  far  that 
possible  loss  of  amalgamable  gold  cannot  be  detected  after 
the  careful  amalgamator,  except  by  the  '  eyes '  of  amalgam 
appearing  in  the  tailing  flume.  Then,  take  the  tailing  to 
the  cyanide  plant  and  do  the  best  that  can  be  done  with 
the  machinery  available. 

The  latest  idea  in  stamp-milling,  outside  of  the  increased 
interest  in  heavy  stamps,  is  to  employ  Chilean  mills  as  in- 
termediate grinders  following  stamps.  With  the  viewpoint 
that  the  tube-mill  finds  its  greatest  efficiency  in  reducing 
30  or  40-mesh  material  to  200-mesh,  the  Chilean  mill  in 
medium  grinding,  and  the  stamp  in  coarse  crushing,  it  is 
proposed  to  use  heavy  stamps  crushing  through  a  4  to  12- 
mesh  screen,  delivering  to  Chilean  mills  grinding  through 
a  30  or  40-mesh  screen  to  tube-mills  sliming  to  the  desired 
fineness.  Without  doubt  the  general,  if  not  universal,  suc- 
cess will  attend  this  method,  but  there  is  the  possibility 
that  it  may  not  in  every  case  be  an  economic  success,  since 
Chilean  mill  grinding  in  the  roll-mill  process  has  a  reputa- 
tion of  being  comparatively  costly. 


CHAPTER  X 

The  men  who  are  in  charge  of  stamp-mills  are  almost 
invariably  good  mill  mechanics,  a  large  part  have  gradu- 
ated out  of  machine  shops,  and  even  the  least  of  them  are 
first-class  '  monkey-wrench '  machinists,  but  only  a  small 
part  are  metallurgists.  The  methods  of  many  of  them  are 
those  that  were  taught  them,  and  these  methods  they  ap- 
ply to  all  conditions  with  but  little  variation.  This  lack 
of  ability  to  initiate  experiment,  to  test,  to  devise  new 
methods,  and  to  progress,  has  hampered  the  advancement 
of  the  stamp-mill  process.  It  is  seen  in  the  tenacity  with 
which  they  cling  to  the  old-time  idea  of  saving  the  maxi- 
mum amount  of  gold  in  the  mortar,  when  the  same,  and 
in  some  cases  higher,  saving  could  be  obtained  by  giving 
the  stamp-battery  a  chance  to  perform  its  proper  function 
— to  prepare  the  ore  for  amalgamation,  rather  than  to 
amalgamate  it.  Wherever  the  millmen  have  forsaken  the 
well-beaten  path  of  trying  to  save  all  the  gold  possible  in 
the  mortar,  the  tonnage  has  increased  and  ease  of  opera- 
tion has  been  promoted.  To  "catch  the  gold  as  soon  as 
you  can  catch  it  inside  the  mortar,"  is  a  good  old  maxim, 
but  the  slogan  of  the  millman  should  be,  ' '  down  with  the 
tailing  and  up  with  the  tonnage,"  and  not,  "increase  the 
inside  catchment — keep  it  up  to  60  or  80  or  95%."  The 
millnum  should  understand  adjusting  the  mill  to  the  pe- 
culiar requirements  of  the  ore,  that  he  may  be  able  in- 
telligently to  put  his  slogan  into  actual  practice.  He 
should  also  be  able  to  determine  the  point  where  a  higher 
tonnage  ceases  to  be  desirable  by  reason  of  resulting  in 


TRAINING  OF  MILLMEN  149 

too  high  a  tailing,  the  economic  limit  having  been  reached. 

The  millman  should  have,  in  addition  to  training  in 
large  and  small  mills  in  various  localities,  and  in  mechani- 
cal work  dealing  in  a  general  way  with  the  setting  up, 
operating,  and  repairing  of  machinery,  with  carpentering, 
pipe  fitting,  and  construction  work,  a  short  training  in 
assaying  and  ore-testing,  and  some  study — home  study  if 
nothing  more — in  chemistry  and  mechanical  and  construc- 
tive drawing.  In  view  of  the  wide  application  of  elec- 
tricity as  the  motive  power  for  mills,  the  millman  should 
understand  the  use  and  care  of  alternating-current  ma- 
chinery. While  it  is  not  expected  that  he  should  be  able 
to  set  up  transformers  or  connect  the  coils  of  motors,  he 
should  understand  more  than  to  merely  start  the  motor 
according  to  the  printed  directions. 

Much  has  been  written  and  said  about  the  honesty  of 
mill  employees.  One  of  the  principal  arguments  advanced 
for  dispensing  with  amalgamation  and  centralizing  the  re- 
covery of  gold  in  the  cyanide  plant,  is  that  it  will  prevent 
loss  of  amalgam  by  theft.  It  has  been  the  fortune  of  the 
writer  to  have  worked  and  associated  with  many  millmen 
in  various  parts  of  the  country,  and  to  have  come  in  con- 
tact with  them  on  an  equal  footing  and  under  conditions 
whereby  their  character  could  be  best  studied,  and  he  has 
not  known  of  a  case  of  amalgam  theft  or  a  suspicion  of 
such,  except  by  report. 

There  are  two  reasons  why  so  little  thieving  occurs,  de- 
spite the  fact  that  amalgam  stealing  appears  easy  and  safe. 
The  first  is  the  esprit  de  corps,  or  loyalty  to  the  profession, 
which  is  as  strong  in  the  millman  as  in  any  other  calling. 
The  second  is  that  the  amalgam  or  bullion  is  viewed  by 
the  millman  as  so  much  merchandise  which  he  is  accumu- 


150  STAMP  MILLING  AND  AMALGAMATION 

lating  for  his  employer,  just  as  he  is  saving  the  sulphide 
in  the  concentrating  department.  It  is  an  actual  fac't  that 
millmen  who  may  'high  grade'  when  working  in  the  mine, 
or  on  the  rock-breaker,  will  take  no  amalgam  from  the 
mill  and  nothing  more  than  a  specimen  from  the  feeders. 

The  danger  of  amalgam  theft  lies  in  putting  a  green 
man  of  unknown  character  in  a  mill  as  helper,  or  tem- 
porarily on  clean-up  day.  Also  in  the  employment  of  a 
so-called  millman  who  is  only  following  milling  until  he 
can  find  an  easier  way  for  getting  the  living  that  'the 
world  owes  him.'  Outside  of  the  above  two,  the  danger 
does  not  lie  mainly  with  the  professional  mill  employee, 
but  with  the  dishonest  manager,  superintendent,  or  con- 
fidential man  who  does  the  melting,  and  who  may  have  a 
private  ingot  mold  of  his  own  to  fill. 

In  late  years  a  new  class  of  stamp-mill  superintendents 
has  arisen,  these  are  the  cyanide  metallurgists,  who  as 
milling  and  cyaniding  operations  are  becoming  more 
closely  linked  together,  are  taking  both  operations  in 
charge.  Where  the  work  is  carried  out  in  conjunction 
this  is  a  step  in  the  right  direction,  but  one  in  advance  of 
the  supply,  for  it  is  difficult  to  find  men  who  have  a  thor- 
ough experience  in  stamp-milling,  amalgamation,  and  cya- 
nidation,  mechanically  as  well  as  metallurgically.  In  the 
extended  acquaintanceship  of  the  writer  there  is  only  one 
past  master  of  stamp-milling,  amalgamation,  and  cyanida- 
tion,  who  is  able  to  direct  and  instruct  his  subordinates 
in  every  detail. 

The  tendency  of  these  new  mill  superintendents  who 
have  little  or  no  training  in  stamp-milling  and  amalgama- 
tion, is  to  put  too  much  stress  on  the  cyanide  branch. 
These  are  the  men  who  would  grind  all  the  ore  so  that  the 


DISCIPLINE  IN  THE  MILL  151 

value  would  be  extracted  by  cyanide  solution,  disregard- 
ing the  fact  that  if  a  grain  of  gold  is  caught  on  the  plates, 
practically  100%  of  it  is  recovered;  whereas,  if  it  goes  to 
the  cyanide  plant,  a  little  of  it  is  lost  through  the  various 
wastes  of  solution,  the  cleaning  up,  and  through  the  im- 
perfect washings  of  the  filters  used. 

The  stamp-battery,  to  do  good  work,  requires  to  be  in 
the  hands  of  a  man  who  is  in  immediate  charge  of  it,  one 
who  is  a  good  millman  and  a  strict  disciplinarian,  a  crank 
on  having  everything  done  right  and  kept  in  condition, 
stopping  just  short  of  the  point  where  the  details  to  be 
carried  out  become  idealistic  rather  than  practical  and 
beneficial.  The  average  competent  mill  employee  prefers 
to  work  under  these  conditions,  rather  than  where  no 
system  prevails  and  everything  is  racked  to  pieces  so  that 
he  must  constantly  keep  a  sharp  outlook  for  trouble  and 
be  continually  repairing.  The  placing  of  a  stamp-battery 
in  charge  of  a  master  mechanic  who  is  not  an  experienced 
millman  and  whose  attention  is  elsewhere  most  of  the 
time,  is  just  as  serious  a  mistake  as  to  consign  it  to  the 
mercy  of  the  different  shifts  of  employees,  all  of  whom 
are  equally  responsible  and  acting  without  a  directing 
head.  A  man  may  be  a  first-class  millwright  and  machinist 
and  still  be  unsuited  by  lack  of  experience  to  take  charge 
of  a  mill.  A  mistake  is  made  in  placing  a  man  in  charge 
of  a  stamp-battery  whose  experience  has  been  superficial, 
no  matter  how  competent  he  may  appear;  the  result  of 
such  error  is  that  the  mill  gradually  wrecks  itself  until 
it  becomes  so  badly  racked  and  worn  and  generally 
broken  down  that  it  is  a  nightmare  for  a  millman  of  long 
experience  to  work  in  it.  The  stamp-battery  is  such  a 
simple  machine  that  an  observing  man  can  learn  to 


152  STAMP  MILLING  AND  AMALGAMATION 

operate  it  under  instruction  in  a  short  time,  but  being 
ponderous  machinery  subjected  constantly  to  jar,  tre- 
mendous vibration,  and  high  tension,  that  to  insure  long 
life  and  good  health  it  must  have  a  man  in  charge  who 
can  promptly  recognize  its  symptoms  of  trouble  and  at 
once  apply  the  proper  remedies. 

The  crew  of  a  10-stamp  mill  having  concentrators  will 
be  composed  of  one  man  per  shift.  The  man  on  the  day 
shift  will  be  in  charge,  and  is  assisted  by  the  man  who 
tends  the  rock-breaker.  A  20-stamp  mill  has  been  run 
by  the  same  sized  crew,  but  the  work  is  so  strenuous  that 
men  will  not  long  remain  and  the  company  suffers  a 
direct  loss  during  their  stay  from  poor  work,  especially 
in  the  concentration.  One  man  per  shift  with  a  head 
millman  can  run  40  stamps  and  do  the  amalgamating ; 
and  for  this  reason  this  is  an  economical  size  of  mill  to 
build.  One  man  per  shift  has  run  up  to  60  stamps  and 
done  the  amalgamating,  but  the  work  is  entirely  too 
arduous  for  one.  The  crew  of  a  100-stamp  mill  will  con- 
sist of  an  amalgamator  in  charge  of  the  shift,  one  battery- 
man  who  attends  to  the  feeding,  and  one  helper.  On  the 
day  shift  there  may  be  a  repair  man  in  addition  to  the 
head  millman.  Should  the  ore  be  low  grade,  requiring 
only  one  or  two  dressings  of  the  plates  in  24  hours,  and 
the  mill  be  in  first-class  condition,  the  helper  on  each 
shift  may  be  dispensed  with.  The  mill  wood  work,  such 
as  making  screen-frames,  chuck-blocks,  and  other  small 
matters  of  this  kind,  are  made  in  the  mine  carpenter  shop. 

Each  5-stamp  battery  is  commonly  designated  by  a  num- 
ber, but  it  is  better  to  use  letters  for  the  batteries,  re- 
serving the  numbers  for  the  stamps  of  each  battery.  Thus 


RECORDS  AND  REPORTS  153 

B4  is  a  short  way  of  designating  in  writing,  or  orally,  the 
fourth  stamp  of  the  second  battery. 

To  record  mill  work  various  report  systems  and  blanks 
are  in  use.  These  should  be  simple  and  cover  the  details 
desired  without  requiring  questions  from  the  management. 
At  some  mills  a  tin  holder  carrying  a  small  sheet  of  paper 
is  nailed  to  the  post  of  each  battery,  upon  which  all  hang- 
ups, breakages,  and  other  causes  of  stoppage  occurring 
to  the  battery  are  noted.  These  papers  are  collected  each 
morning  by  the  mill  foreman  and  turned  into  the  super- 
intendent's office.  An  excellent  method,  especially  in  a 
small  or  medium-sized  mill,  is  to  post  a  form  covering  a 
month  near  the  change  room  of  the  mill.  This  form  is 
on  heavy  detail  paper  and  has  a  line  for  each  shift,  with 
large  space  under  the  caption,  ' remarks.'  Just  before 
going  off  duty,  the  millman  whose  shift  is  ending  fills 
out  his  line,  and  under  'remarks'  notes  down  what  stems 
have  broken,  where  new  steel  has  been  put  in,  what  boxes 
are  running  hot,  and  any  other  details,  so  that  the  on- 
coming millman,  by  glancing  over  the  sheet,  will  note  at 
once  what  has  been  done  on  the  other  shift  and  knows 
what  parts  of  the  mill  require  special  watching.  At  the 
end  of  the  month  the  columns  are  totalled  for  the  monthly 
report  of  operations  and  the  sheet  filed  away  as  a  sum- 
mary of  the  work  for  the  month.  Where  there  is  irregu- 
larity in  the  hours  worked,  or  the  crew  is  large,  the  time- 
slip  method,  whereby  each  man  turns  in  his  own  time, 
should  be  used.  A  ruled  form  in  a  book,  or  posted  on  the 
wall,  should  be  provided  in  which  to  record  supplies  re- 
ceived, used,  and  remaining  on  hand  at  the  end  of.  the 
month. 

For  recording  the  loss  of  running  time  the  'stamp-hour' 


154  STAMP  MILLING  AND  AMALGAMATION 

system  is  the  simplest  and  best.  In  this  system  the  length 
of  time  any  number  of  stamps  is  hung  up  is  multiplied 
by  the  number  of  these  stamps,  the  result  being  called 
'stamp-hours.'  The  idea  is  to  show  the  time  lost  as  meas- 
ured on  one  stamp  only,  and  to  simplify  the  recording  of 
lost  time.  Thus,  on  one  shift  a  single  battery  is  shut 
down  for  20  minutes,  which  is  equivalent  to  one  stamp 
being  shut  down  for  100  minutes  or  1%  stamp  hours; 
later  on  10  stamps  may  be  shut  down  for  30  minutes, 
making  a  loss  of  5  stamp-hours,  or  a  total  of  6%  for  the 
shift.  At  the  end  of  the  day,  or  month,  the  millman 
divides  the  total  number  of  stamp-hours  lost  by  the  num- 
ber of  stamps  in  the  mill,  and  the  result  is  equivalent  to 
the  number  of  hours  of  running  time  lost  by  the  entire 
mill. 

The  mill-foreman  should  also  be  provided  with  a  blank 
form  in  which  he  should  enter  daily  the  following  data : 
Number  of  tons  crushed ;  ounces  of  mercury  fed  inside  the 
batteries,  on  outside  plates,  and  ounces  amalgam  collected 
from  outside  plates.  These  data  relating  to  feeding  of 
silver  and  collection  of  amalgam  should  be  entered  for 
each  unit  of  5  stamps,  if  accurate  information  is  desired ; 
also  number  of  pounds  (wet  weight)  of  sulphides  col- 
lected from  concentrators  each  24  hours.  To  this  sheet 
may  properly  be  added  the  various  stoppages  and  their 
cause,  such  as  dressing  plates,  broken  belts,  babbitting 
shafts,  changing  screens,  broken  stems,  replacing  shoes  or 
dies,  power  off,  and  the  numerous  other  affairs  that  inter- 
fere with  the  steady  operation  of  the  mill.  This  sheet  is 
not  posted  for  general  inspection,  but  goes  promptly  each 
morning  to  the  office  of  the  superintendent,  where  it  is 


DAILY  REPORTS  155 

entered  on  a  book  kept  for  the  purpose,  and  the  mill- 
sheet  placed  on  file. 

Another  sheet  should  show  the  supplies  consumed,  in- 
cluding shoes,  dies,  screens,  quicksilver,  lubricants,  light, 
belting,  chemicals,  lumber,  water,  power  (read  by  meter), 
concentrator  belts,  and  all  other  items  going  to  make  up 
the  cost  of  milling,  including  labor. 

Whether  a  daily  mill  report  is  made  or  not  for  the 
superintendent  at  the  property,  or  the  manager  at  the 
general  office,  a  monthly  summary  of  operations  should 
always  be  required  of  the  mill  superintendent,  which 
should  include  a  cost-sheet,  and  also  a  description  of  all 
tests  and  experiments  made.  This  summary  should  be 
exhaustive,  giving  all  the  data  of  any  practical  value  it  is 
possible  to  obtain,  and  these  should  become  a  part  of  the 
permanent  records  at  the  property,  with  a  duplicate  at 
the  general  office.  In  the  preparation  of  this  report  the 
millman  will  observe  many  things  of  interest  and  value 
that  may  result  in  further  study  and  increased  efficiency. 
It  would  require  too  great  a  length  to  speak  of  the 
invaluable  uses  these  reports  are  put  to.  A  concrete 
illustration  will  suffice.  A  small  mill  having  a  some- 
what complicated  treatment  system  was  operated  for  an 
extended  period  by  different  metallurgists  of  repute.  The 
mill  finally  shut  down  pending  negotiations  for  equipping 
the  property  with  a  larger  plant,  and  the  blocking  out  o'f 
ore.  When  it  was  decided  to  begin  metallurgical  opera- 
tions on  an  increased  scale,  the  company  in  attempting  to 
decide  whether  to  increase  the  small  plant,  build  a  larger 
mill,  or  use  some  other  system  for  treating  the  ore,  found 
itself  with  only  a  lot  of  scattered  incomplete  information 
of  the  most  vague  nature  much  of  which  was  hearsay.  In 


156  STAMP  MILLING  AND  AMALGAMATION 

this  extremity  they  were  obliged  to  send  out  a  metal- 
lurgist to  start  up  the  small  plant  and  by  a  series  of  ex- 
periments determine  what  could  be  done — in  short,  to  get 
the  data  that  a  proper  report  system  should  have  given. 

The  general  superintendent,  or  manager,  can  materially 
assist  milling  operations  by  impressing  on  the  mine  fore- 
man the  necessity  for  keeping  the  mill  bins  full ;  by  keep- 
ing the  mine  foreman,  the  mill  foreman,  the  cyanide  man, 
and  the  assayer  in  harmonious  relation  instead  of  antag- 
onistic to  each  other,  as  is  too  often  the  case ;  and  by  urg- 
ing the  mill  foreman  to  take  advantage  of  the  help  of 
the  assayer  in  his  testing. 

Directly  in  front  of  the  middle  of  the  batteries  a  floor 
should  be  erected  overhanging  the  concentrator  floor.  A 
good  stove  should  be  placed  here,  that  the  batteryman 
on  the  night  shift  may  be  able  to  warm  himself  at  a  point 
where  he  has  everything  in  plain  sight,  instead  of  going 
out  to  the  boilers,  or  down  to  the  cyanide  plant.  In  a  cold 
climate  these  floors  have  been  boarded  up  to  form  a  clean 
and  cozy  change-room  with  a  glass  front.  The  clean-up 
room  can  be  situated  to  advantage  at  this  point.  It  is 
preferable,  where  not  too  cold,  to  surround  it  with  wire 
netting  instead  of  boarding  it  up,  that  there  may  be 
more  light.  It  is  a  wise  expenditure  to  build  a  commo- 
dious and  well-equipped  clean-up  room.  Floors  should 
be  built  tight  and  drain  into  a  launder  running  the  length 
of  the  mill  to  a  box  from  which  the  amalgam,  sulphide, 
and  sand  that  has  been  flushed  into  it  can  be  recovered 
to  go  into  the  clean-up  barrel  or  through  the  mortars. 
Concrete  floors  make  a  neat  looking  mill,  but  are  cordially 
detested  by  mill  employees  as  they  produce  leg-weariness, 
calloused  feet,  and  'draw  the  cold  and  dampness/  A  mill- 


TOOLS  AND  REPAIRS  157 

man  experiences  a  great  relief  in  changing  from  a  mill 
having  concrete  floors  to  one  having  wood.  The  use  of 
rubber  heels,  overshoes,  or  thickly  soled  shoes  affords 
some  relief.  Where  concrete  floors  are  put  down,  a  walk 
of  plank  or  grating  should  be  run  between  the  concentra- 
tors and  along  in  front  of  the  plates  for  the  benefit  of  the 
employees. 

A  work-bench  should  be  provided  together  with  a  full 
set  of  the  tools  such  as  are  required  in  a  stamp-mill,  in- 
cluding the  more  common  carpenter  and  machinist  tools. 
These  tools  should  be  stamped  but  not  kept  under  lock 
and  key,  with  the  exception  of  the  extras.  Each  tool 
should  have  a  place  and  the  name  of  the  tool  should  be 
printed  at  that  place.  During  shut-downs  on  account  of 
lack  of  power,  or  other  external  causes,  the  belts  should 
be  gone  over  and  those  parts  of  the  mill  repaired  and 
cleaned  that  cannot  be  conveniently  gotten  at  while  in 
operation. 

The  millman  who  wishes  to  make  a  name  for  himself 
will  keep  his  mill  scrupulously  clean  and  neat  in  appear- 
ance. He  will  first  stop  the  running  out  and  splattering 
about  of  oil  and  grease,  likewise  of  the  water  and  pulp, 
then  of  the  ore  that  falls  out  of  the  chutes  and  about  the 
feeders.  Finally  he  will  remove  the  rust,  grease,  and  mud 
from  the  stems  and  shafts,  and  the  mortars.  Once  the 
mill  is  burnished  up  and  the  splattering  about  and  leakage 
stopped  the  mill  can  easily  be  kept  in  this  condition.  A 
clean  mill  and  a  good  millman  go  together.  There  is  a 
great  difference  in  millmen,  some  are  so  careless  and  in- 
expert in  making  repairs  and  so  unsystematic  in  the  daily 
routine  work  that  the  duties  become  trying  to  the  em- 
ployees. Other  millmen  are  able  to  keep  their  mills  in 


158  STAMP  MILLING  AND  AMALGAMATION 

such  good  condition  and  to  plan  the  routine  of  daily  work 
so  well  that  the  mill  work  is  no  longer  odious  but  >  is  ac- 
companied with  a  considerable  degree  of  gratification.  Be- 
sides an  able  millman,  a  well  constructed  and  properly 
designed  mill  is  necessary.  With  these  factors  the  stamp- 
mill  becomes  the  most  satisfactory  metallurgical  machine 
in  use,  to  those  both  directly  and  indirectly  interested. 
There  are  40-stamp  mills  operating  without  a  machine- 
shop  or  even  a  lathe,  all  repairs  necessary  being  made 
by  the  millman  sometimes  assisted  by  the  blacksmith. 
There  are  10-stamp  mills  that,  figuratively  speaking,  are 
in  the  machine-shop  all  the  time,  due  to  the  absence  of 
one  of  these  factors,  usually  to  defects  in  the  installation. 

No  whistling  or  shouting  should  be  allowed  in  a  mill, 
except  as  a  danger  signal.  To  call  attention  a  hissing 
noise  should  be  made ;  as  such  a  noise  is  keyed  in  a  differ- 
ent pitch  from  that  of  the  stamps,  it  can  be  heard  across 
a  large  mill.  Similarly,  in  talking,  no  attempt  should  be 
made  to  talk  above  the  roar,  but  in  a  moderate  tone 
keyed  in  a  different  pitch,  which  can  only  be  learned  by 
experimenting.  Sign  language  should  be  developed  as 
far  as  possible.  Where  it  is  desired  to  call  a  man  at  a  dis- 
tance, as  at  the  rock-breaker  or  the  cyanide  plant,  a  tri- 
angle, such  as  is  in  common  use  at  mine  boarding-houses, 
should  be  used.  In  some  mills  the  foreman  carries  a  po- 
lice whistle  for  the  purpose  of  calling  men.  Colored  sig- 
nal lights  are  used  for  telephones. 

There  is  a  serious  part  of  stamp-milling — the  loss  of 
hearing.  A  10  or  20-stamp  mill  is  not  hard  on  the  hear- 
ing, but  the  larger  mills  cause  the  majority  of  men  to  be- 
come deaf  in  time.  To  save  the  ear  drums  as  much  as 
possible  and  reduce  the  distress  of  the  continuous  noise, 


ELEVATING  PULP  159 

wads  of  cotton,  wool,  or  waste  are  worn  in  the  ears.  These 
should  be  softened  with  clean  oil  such  as  olive  oil  or 
vaseline,  that  they  may  not  inflame  the  ears.  Further 
relief  can  be  obtained  by  sealing  the  ears  up,  after  the 
cotton,  with  a  soft  pliable  wax  or  stiff  salve  that  can  be 
moulded  into  place. 

For  elevating  pulp  where  necessary  the  tailing  wheel  is 
used  at  large  installations.  As  these  wheels  are  costly  to 
install  and  the  cost  of  operating  remains  practically  the 
same  whether  a  large  or  small  quantity  of  pulp  is  to  be 
elevated,  some  other  machine  is  used  in  small  plants.  The 
centrifugal  pump  is  too  costly  and  troublesome  from  the 
shell,  liners,  and  stuffing-box  of  the  pump  becoming  rap- 
idly worn  by  the  grit.  The  Fernier  sand-pump  has  been 
found  very  satisfactory.  Its  maximum  lift  is  about  20 
ft.  in  a  practical  way,  and  for  a  higher  lift  two  or  more 
of  them  should  be  placed  tandem.  The  wear  on  these 
pumps  is  mainly  in  the  bearings,  the  grit  of  the  pulp  giv- 
ing no  trouble.  They  have  to  be  stopped  for  a  period  of 
10  minutes  every  2  to  4  weeks  to  allow  replacing  the  gland 
with  one  that  has  been  re-packed.  Experiments  have  been 
made  with  an  air-lift  pump.  These  cost  little  to  install 
and  to  keep  in  repair,  but  their  efficiency  is  low.  The  air- 
displacement  pump  is  now  being  used  in  pumping  slimy 
and  gritty  liquids  and  slush,  and  is  being  introduced  for 
pumping  thickened  pulp  into  filter-presses  against  a  high 
head.  This  pump  would  appear  to  solve  the  problem  of 
a  cheap  and  efficient  installation  for  elevating  wet  pulp 
as  they  cost  but  slightly  more  than  a  centrifugal  pump, 
require  little  attention,  are  compact,  have  high  efficiency, 
and  the  cost  for  wearing  parts  or  repairs  is  nominal.  The 
hydraulic  lift,  or  elevator,  is  being  used  'for  elevating  pulp 


160  STAMP  MILLING  AND  AMALGAMATION 

where  the  extra  amount  of  water  necessary  to  their  opera- 
tion is  not  considered  undesirable,  as  in  a  canvas-plant. 

Stamp-mill  launders  within  the  mill  should  be  set  with 
a  grade  of  one  foot  in  twelve.  Discharge  launders  or 
flumes  leading  away  from  the  mill  should  have  a  grade 
of  not  less  than  one  foot  in  sixteen.  A  grade  of  one  foot 
in  twenty  will  work  under  favorable  conditions  without 
giving  trouble,  but  is  generally  too  small,  particularly 
in  a  cold  country.  In  one  case  the  finely  crushed  tailing 
of  a  40-stamp  mill  was  transported  in  a  flume  having  a 
grade  of  one  foot  in  thirty-two,  but  considerable  trouble 
was  experienced. 

Where  it  is  necessary  to  impound  the  tailing,  it  is  usu- 
ally done  to  prevent  it  from  reaching  sites,  or  water 
courses,  where  it  is  not  wanted,  or  for  the  purpose  of  sub- 
sequent treatment,  such  as  by  the  cyanide  process.  A 
hillside  can  usually  be  had  to  form  a  wall  on  from  one  to 
three  sides,  while  the  pond  is  laid  out  in  two  or  more  sec- 
tions that  the  walls  of  one  section  may  be  raised  by  shov- 
eling or  scraping  while  the  other  is  filling.  Where  the 
tailing  is  not  being  banked  for  future  treatment,  the  tail- 
ing flume  is  carried  over  the  pond  just  inside  of  the  wall 
that  has  to  be  raised  by  shoveling.  The  tailing  is  dis- 
charged along  this  wall  through  several  small  gates.  The 
coarser  sand  of  the  tailing  settles  at  this  point,  damming 
the  slime  back  against  the  hillside  where  a  box  flume  laid 
on  the  ground  and  passing  underneath  the  pond,  carries 
off  the  clearer  water.  The  depth  of  the  slime  is  increased 
by  extending  the  bedrock  flume  as  needed.  The  mill 
flume  can  run  along  the  contour  of  the  hill  and  small  V- 
shaped  troughs  can  be  lightly  trestled  up  to  carry  the  tail- 
ing to  the  outer  bank  of  the  pond.  This  method  of  filling 


RECOVERY  OP  WATER  161 

results  in  the  slime  being  separated  from  the  sand,  and 
should  not  be  used  where  the  tailing  is  to  be  eventually 
cyanided,  as  it  is  impossible  to  treat  this  caked  slime  with- 
out a  highly  expensive  equipment.  Four  sets  of  inlets 
and  outlets  should  be  spaced  about  each  section  of  the 
pond  and  a  change  made  from  one  set  to  another  daily  or 
semi-daily.  This  will  result  in  throwing  a  layer  of  sand 
upon  a  stratum  of  slime  through  which  it  will  sink  to  some 
extent,  and  will  enable  the  cyanide  man  to  put  a  homo- 
geneous and  leachable  charge  into  his  tanks,  without  leav- 
ing any  material  behind  as  unleachable. 

These  ponds  have  been  used  to  return  some  of  the  water 
to  the  mill  for  re-use,  but  a  settling  plant  is  much  better. 
This  may  be  of  the  well  known  cone  settlers  purchased 
outright  or  home-made  of  wood  at  a  small  plant.  A  simple 
and  efficient  water-saving  plant  consists  of  deep  pulp- 
thickening  tanks  ending  in  cones  with  60°  sides  and  large 
gate-valve  discharges.  The  pulp  is  introduced  into  the 
center  of  the  tank  some  distance  below  the  surface  of  the 
overflowing  water  that  its  introduction  may  not  be  violent 
and  that  the  settling  effect  of  a  slowly  ascending  column 
of  water  may  be  obtained.  The  valves  are  opened  at  in- 
tervals of  a  few  hours  to  withdraw  a  part  of  the  thickened 
pulp,  which  comes  out  as  a  thick  and  solid  sludge  requir- 
ing a  launder  set  at  heavy  grade.  The  wooden  box  set- 
tlers so  common  around  small  mills  on  the  desert  and  so 
unsatisfactory  in  operation,  should  be  divided  into  com- 
partments fitted  with  these  cone  bottoms  and  gate  valves. 

Novel  instances  of  mill-tailing  put  to  agricultural  pur- 
poses are  found  in  California.  Some  30  years  ago,  the 
tailing  from  a  large  mill  crushing  a  quartz  ore  containing 
much  of  the  slate  in  which  the  orebody  lay,  after  run- 


162  STAMP  MILLING  AND  AMALGAMATION 

ning  in  a  creek  for  three  miles,  was  diverted  by  an  earth 
dam  and  impounded  to  form  a  fill.  The  flat  bottom  of  the 
creek  had  been  cleaned  bare  years  before  by  placer  min- 
ers. A  stone  wall,  6  by  15  ft.  high,  was  roughly  laid  at 
the  side  of  the  creek,  to  raise  the  proposed  surface  above 
high  water.  This  wall  was  about  2200  ft.  long,  and  in- 
closed a  strip  of  ground  of  that  length  and  from  125  to  250 
ft.  wide,  to  be  filled  by  the  tailing.  This  filling  was  done 
in  sections  and  required  several  years  to  complete.  The 
surface  was  well  manured,  and  the  tailing  was  found  to 
make  an  excellent  soil  for  growing  vegetables  and  fruits, 
being  preferred  to  the  natural  soil.  It  is  still  actively 
worked  and  is  considered  the  best  garden  in  the  county. 
There  are  many  gardens  of  this  character  along  the 
Mother  Lode  of  California. 


INDEX 


Page. 
Adaptability  of  Stamp-Mills    10 

Adjusting  Dies    24 

Adjustment   of  Drop 44 

Of    Stamps    15 

Aerial  Tramways    12 

Affinities   of   Amalgam 88 

Air  Bubbles  on  Plates 109 

All    Sliming    145 

Amalgam  Dope    114 

Properties  of    74 

Amalgamation     71 

Tests     138 

Amount   of   Water 57 

Anchor  Bolts    20 

Angle   of  Screens 56 

Annealing   Crucibles    124 

Stems    38 

Apron   Plates    82 

Assay   Furnace   for   Retort- 
ing     123 

Automatic   Samples    141 

Babbitting     47 

Back-Knee    Construction...    17 

Banking   of   Pulp 79 

Bare   Spots    113 

Spots   on    Plates 78 

Battery   Construction    17 

Foundations     19 

Samples    141 

Water    Temperature 106 

Bins    14 

Black    Hills   District 146 

Blacksmith    Forge    for   Re- 
torting     123 

Blake  Crushers    13 

Blankets     96 

Blue    Plates    83 

Borax  Glass  as  Flux 125 

Boss-Heads    30 

Weight     61 

Brass    Screens    51 

Breaking  Stems   36,  41 


Page. 

Burning  Plates    106 

Burnishing  Plates    104 

California  Practice  94 

Cam  Shafts  45 

Sticks  34,  55 

Camming  of  Stamps 46 

Capacity  of  Stamps 60,  69 

Cast  Dies  25 

Challenge  Feeders  50,  55 

Chemicals  on  Plates Ill 

Chilean  Mills.. 62,  144,  146,  147 
Chuck  Block  76 

Block  Plate  22 

Cleaning  Amalgam  ' 116 

Plates    116,  118 

Up   85,  93,   118,   156 

Up  Barrel  119 

Cleanliness  in  Milling 157 

Clogging  Screens  52 

Coarse  Gold  91,  99 

Coating  Moulds 127 

Color  of  Sponge 123 

Concentrate  Piping  13 

Concrete  Battery  Blocks...  19 

Cone  Settlers  161 

Contaminated  Mercury  ...113 

Construction  of  Tables 95 

Copper  in  Amalgam.  .113,  115 

Plates  77,  101 

Cost  of  Installation  10 

Of  Shoes  25 

Crew  for  Milling  152 

Crucibles,  Annealing  of 124 

Crude-Ore  Bins  14 

Crushers  13 

Crushing  in  Cyanide 142 

In  Solution  ..143 

Crystallization  of  Stems...  37 

Cupping 26 

Cushioning  26 

Cyanide  and  Amalgamation. 100 

And  Quicksilver    73 


164 


INDEX 


Page. 

Metallurgists    150 

On  Plates 104,  111,  114 

Deafness  158 

Demurrage  154 

Design  of  Cams 35 

Of  Tables 96 

Dies  24,  25 

Discharge  Lip  22 

Discipline  in  Mill 151 

Disposition  of  Tailing.  ...  13 

Distributing  Box 82,  92 

Double  Discharge  Mortars.  23 

Discharge  Stamps  68 

Dressing  Plates 85,  89,  93 

Plates,    Frequency   of....  107 

Stems  41 

Driving  Shoes  27 

Drops  95 

Dry  Amalgamation  89 

Plates  83 

DutchVnen  27 

Duty  of  Stamps 60,  69 

Economic    Problems    144 

Effect   of   Sodium 115 

Elevating  Pulp 159 

Elevators  in  Mills 12 

Empire  Mill    94 

Eyes  of  Amalgam... 147 

False    Dies    23 

Shoes     61 

*  atigue   of  Metal 37 

Feed  Chute  from  Bins 17 

Water     57 

Feeders     50,  55 

Feeding   Battery    75 

Mercury     75 

Fernier   Pumps    160 

Finger  Jacks   55 

Float   Gold    130,  131 

Flouring   of    Quicksilver...    72 

Flux   in  Retorting 124 

Free  Gold    130 

Freezing  Amalgam  to  Iro^n.115 
Frequency       of       Dressing 
Plates     .  ..107 


Page. 

Friction     .    65 

Front-Knee    Construction..    18 

Galvanic   Effect   on   Plates.  113 

Gaskets  on  Screens 56 

Gibs 31 

Gilpin    County   Practice.... 

45,   63,  91,   131,   134 
Gold   in    Sulphides 131 

Pan   for  Testing 140 

Grade    of   Launders 160 

Of  Ore,   and   Plates 88 

Graphite   in   Ore 133 

Grass   Valley   Practice 94 

Grease   in   Ore 133 

Treatment    of    119 

Grizzlies     14 

Grouting  Mortar  Blocks...  19 
Guides  55 

Hard   Amalgam 84,    108 

Head   Assays    136 

Heating   Mill    156 

Height  of  Chuck  Block....    76 

Of  Discharge    62 

Of  Drop    15,   43,  59 

Helpers   in    Cleaning  Up... 150 

Homestake   Mortars    22 

Practice    110 

Honesty   of   Millmen 149 

Horse-Power    for    Stamps..    65 

Impounding  Tailing.  .  .144,  160 
Impurities  in  Amalgam.  ..  .125 

In  Ore ..130,  135 

Individual  Stamps  67 

Inside  Amalgamation.  .  .22,  79 

Plates 76,  77 

Iron  in  Amalgam 125 

Jar  of  Battery  Posts 47 

Of  Tables  94 

Jaw  Crushers 13 

Labor    in    Mills 152 

Lake    Superior    Copper    for 

Plates    102 

Lateral  Thrust  of  Cams...  35 
Launders  .  ..97,  160 


INDEX 


165 


Page. 

Left-Hand  Cams  35 

Length  of  Plates 91 

Line  Shaft  17 

Lining  Mortars  23 

Lip  Plate  81 

Loading  Amalgam  114 

Losses  in  Amalgamation.  .  .130 

Loss  of  Mercury  86 

Low  Discharge  24 

Grade  Ore  93 

Lubrication  of  Cams 45 

Lye  on  Plates 104 

To    Cut    Grease 119 

Matte    from    Retorting 126 

Mechanics    in    Mill 148 

Melting    Retort    Sponges.  .  .126 

Mercury     71 

Metallurgists 150 

Millmen    148 

Millsites    11 

Mill    Superintendents    150 

Tests     136 

Wrights     151 

Mortar  Blocks    19 

Liners    23 

Mortars     22 

Mother   Lode  District.  .141,  162 
Mould  for  Retorting 127 

Nitre  for  Retorting 126 

Nitric  Acid  on  Plates 104 

Noises   in   Mill 158 

Novelty  Mills   147 

Oilers    18 

Order   of   Drop 49,  53 

Ore   Bins    14 

Feeders    50,  55 

Reserve    16 

Outside    Amalgamation     ...    98 

Over   Feeding    64,  79 

Feeding  Mercury    83 

Stamping 130,    131,    134 

Patent    Amalgamators    98 

Percentage    of   Recovery.  .  .123 
Piping  Concentrate  ' 13 


Page. 

Plate    Table 94 

Plumbing   Dies    24 

Poling   Amalgam    125 

Poor  Amalgamation. .  .130,  135 

Pounding    26 

Stamps     39 

Power  Necessary    65 

Preliminary  Tests    138 

Preparation  of  Plates 103 

Preparing     Sodium     Amal- 
gam     114 

Pulp,    Elevation   of 159 

Purification  of  Quicksilver. 

73,  113 
Putting  on  Shoes 28 

Quadruple-  Discharge 

Stamps    68 

Quicksilver 71 

Raw  Copper  Plates 102 

Records    153 

Recovery   in    Mortar 79 

Of   Water    161 

Percentage   of    123 

Regrinding     144 

Removing  Shoes   27 

Reports     153 

Retorting   Amalgam    121 

Right-Hand   Cams    35 

Roads    to   Mill 13 

Rock   Breakers    13 

Roof  of  Mill   13 

Rotation    of    Stamps 45 

Rubber   for  Plates 90 

Rubbing    in   Mercury 85 

Rusty   Gold    130,   133 

Salting  Samples    . 142 

Sampling     136,  141 

Bars     . 128 

Sand   in   Amalgam 125 

Scouring  by   Pulp 76 

Screws    50 

Self-Tightening    Cams    48 

Setting   Tappets    33 

Shoes    .  .    25 


166 


INDEX 


Page. 

And   Dies,    Cleaning 129 

Socket     61 

Weight    61 

Short   Plates    91 

Shoveling  Ore    13 

Sickening   of    Quicksilver..    72 

Sign   Language    158 

Signals   in   Mills 158 

Silver  Amalgam    115 

Plates     77,  113 

Simplicity   of   Stamps 9 

Sizing  Tests    134 

Slag    from    Amalgam.  .126,   128 

Slime  Ponds    160 

Sliming     145 

In   Stamp  Mills 70 

Slimy  Ores    135 

Slipping   Tappets    31 

Slope   of  Bin   Floor 15 

Sluice   Plates    96 

Sodium  Amalgam    114 

And  Quicksilver    73 

Softness    of    Plates 78,  83 

Soft  Spots  in  Shoes 26 

Solution,   Crushing  in 143 

South  African  Practice.  ..  9,   60 

Speed   in   Tube-Mill 145 

Splash   of   Pulp 64 

Plate     81 

Sponge  of  Amalgam. .  .121,   123 

Spray    on    Table 92 

Stained  Plates    113 

Stamp   Duty 9,   60,  69 

Hour   Duty    153 

Starting  Plates    103 

Stealing  Amalgam    149 

Steel  Dies    25 

Steel  in  Mortar 44 

Stem  Guides   55 

Weight     61 

Stems    36 

Sticking     of     Amalgam     in 

Retorts    121,  123 

Sticky    Ore    14 

Strain    in    Stems 38 


Page. 

Sulphides,  Gold  in 4.  ...131 

In    Amalgam    125 

Ores    93 

Superintendents     150 

Supplies',  Records  of 155 

Sweating  Plates    90 

Tables    83 

Tail   Box 99 

Tailing,  Disposition  of 13 

Impounding     160 

Pond    144 

Treatment     143 

Wheels     159 

Talcose   Ores    135 

Tappet   Keys    30 

Tappets     30,  32 

Weight    61 

Tarnished   Plates    114 

Temperature  of  Water 106 

Tests  of  Mill 136 

Tinned-Iron   Screens    51 

Tonopah    District    ....141 

Tools 157 

For  Dressing 110 

Training   of  Millmen 149 

Treasure  Box   97 

True   Dies    24 

Tube-Mills 62,    145,    146 

Verdigris    113 

Volatilizing   Mercury. 122 

Water    for    Stamps 57,  69 

Recovery    161 

Temperature  of   106 

Wave  Motion  of  Pulp 109 

Wear   of   Shoes 25 

Wedging  on   Shoes 27,   28 

Weight  of  Stamps 60 

Wet  Amalgamation    90 

Wide   Mortars    22 

Wood  and  Chips,  Cleaning.  128 

Wooden   Mortar  Blocks 21 

Work  Bench    .  157 


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